Epic has announced that the Unreal Engine 4.25 Preview release is now available on the Launcher and GitHub. They are making this Preview available for developers to try new features and catch issues before the final release.

The Preview release includes:

AI Updates:

• Navigation Improvements (Beta). The following improvements have been implemented for navmeshes:
  • Options for handling offline navmesh builds.
  • New CostLimit property in FindPath navigation queries.
  • Options for filling collision under navmeshes. When set on a static mesh component, the navmesh will not be generated under the surface of the geometry.

Animation Updates:

• Animation Insights (Experimental). The editor now includes Animation Insights to visualize gameplay state, live animation behavior, and information from recorded traces.
• Animation Timeline Refactor (Beta). Asset editors for animation assets now use a Sequencer-like timeline that provides a more consistent look and feel when dealing with animation data. Editing of Animation Montage sections is now done in a dedicated tab with breadcrumb trails and dropdown menus. Additionally, Animation Curves now use a fully-featured Sequencer-like Curve Editor.
• Animation Compression Improvements (Beta). Plugins can now specify new Animation Compression schemes and compression data structures. Animation Sequences now reference Compression Settings Assets, which contain one or more codecs to apply to the Animation Sequence, replacing the earlier concept of hardcoded “Automatic Compression.” Animation compression is now performed asynchronously as a non-blocking operation inside Unreal Editor.
• Control Rig Improvements (Experimental). Control Rigs are now more memory efficient and performant than the previous iteration (up to 20% faster and up to 75% memory savings). We have added Visual Debugging, Direct Manipulation for values to improve the overall user experience. The hierarchy now supports trees of bones, spaces and controls where controls represent the 3d user interface to the rig. We also improved flexibility in user interface design and added full coverage for scripting in Python to automate workflow tasks as well as extended support for Control Rig in Sequence.

Audio Updates:

• Convolution Reverb (Beta). Convolution Reverb provides a way to digitally simulate reverberation based on real-world samples from physical spaces, and produces a more realistic sound environment.
• Native Soundfield Ambisonics (Beta). This feature brings a new dimension to audio in a 3D, interactive environment by providing sound relative to the position of both the listener and the sound origination point.
• Soundfield and Endpoint Submixes (Beta). These submixes reduce development time when implementing any spatial audio format in UE, and provide better control on spatial localization for immersive gameplay.
• Submix Assetization (Beta). Submix Assetization makes it possible to define default submixes in UE audio settings, providing faster iteration time.
• Master/Sidechain Compression (Beta). This is an improved method for ducking ambient sounds when the listener's focus needs to be heightened. For example, when an enemy approaches, the sound of their footsteps isn't masked by a louder but more distant grenade explosion.

Core Updates:

• Unreal Insights (Beta). This release features UX and architecture improvements for the Insights application performance profiling tool.
• FProperty Refactor. UProperty are refactored to be FProperty, which means that UProperty is no longer a UObject after this release. Note that this is an API update, and the refactor should go unnoticed by end-users who don't work with the source code.

Datasmith Updates:

• LiDAR Point Clouds: Unreal Engine now offers native support for the most popular point cloud formats. We’ve taken the LiDAR Point Clouds Plugin from the Marketplace, made major improvements to its performance and scalability, added new settings and features including the option to delete hidden points, and made the Plugin available directly in the Plugins window of the Unreal Editor.
• Visual Dataprep (Beta). Updates include:
  • The Visual Dataprep graph editor now has a distinct look and feel that better reflects its execution and data flows.
  • We’ve exposed the main Dataprep actions to Blueprint and Python, so your editor scripts can drive imports through the Visual Dataprep system.
  • We’ve added some new Select By nodes to filter objects by Layer, by vertex count, or by triangle count.
  • Several new Operator nodes offer additional options for setting up your scene and your Assets, such as the ability to add random offsets to the 3D positions of your Actors, flip the facing directions of triangles in your Static Meshes, replace Asset references, and more.
• Datasmith Interop. Several updates have been made, including:
  • Rhino: You can now choose on import whether to have Datasmith tessellate all parametric surfaces for you, or whether you want to reuse triangular meshes previously created by Rhino and stored in your Rhino scene.
    In addition, Datasmith now imports Rhino points as empty Actors, and it imports technical metadata about Rhino objects as Tags on the Actors.
  • Revit: The Datasmith Export Plugin for Revit now includes a plugin for Dynamo: DatasmithDynamoNode.dll. It contains a new DatasmithDynamoNode.Export3DViewsToDatasmith node that you can use to batch export multiple 3D views from Revit to .udatasmith files.
    We’ve also made many fixes and improvements in handling pivot points, importing families and hierarchies, converting PBR materials, and naming Static Mesh Components and Texture Assets.
  • PLMXML: The Datasmith CAD Importer can now import PLMXML files. The import process automatically imports geometry from referenced files, preserves scene hierarchy and placement, converts PLMXML User Data to Datasmith Metadata, and creates a Variant for each PLMXML Product View that you can use with the Variant Manager.
• Variant Manager. We’ve made several ease-of-use improvements to the Variant Manager UI:
  • You can use external image files as thumbnails for Variants.
  • Automatically swap a bound Actor in a Variant to a different Actor.
  • Reorder captured properties.
  • Set the values of captured color properties more easily using a color picker.
  • Capture settings from Cine Camera Actors.
  • When you set up a Variant to call a Blueprint function on activation, the function now receives additional arguments for context: the Level Variant Set Asset, the Variant being activated, and the Variant Set that owns the activated Variant. This context helps you set up complex responses, such as activating other Variants that are dependencies for the one being activated.
• Editor Performance (Experimental). This release makes significant performance gains when saving newly created Assets, creating proxy meshes, decomposing geometry into convex hulls, and using Datasmith to import from .udatasmith, Deltagen, and VRED.
• Modeling Tools (Experimental). We continue to make improvements to our experimental in-viewport geometry editing and sculpting tools. This release focuses on adaptive mesh sculpting, interactive tools for creating and modifying polygroups and UV mappings, and mesh repair tools.
• Live USD (Experimental). The USD Importer is now faster and more responsive, with multi-threaded import and better rendering performance. It also makes your scene integrate better into Unreal workflows — for example:
  • Each USD Layer is brought in with its own Sequencer track.
  • The importer brings in the USD Purpose of your scene objects (default, render, proxy, guide).
  • You can assign Unreal Materials to your USD Primitives.
  • The USD Stage is now exposed to Blueprints and to Python, so you can control it from editor scripts that you write.
  • If you have a custom USD schema, you can hook up custom C++ callback functions to handle your own data structures.

Editor Updates:

• Disaster Recovery. The Data Recovery system records transactions and sends them to a recovery service. When the Editor detects that a session has ended abnormally, it sends a request to the recovery service and retrieves the last recorded session.
• Editor/Paint Mode Refactor. The Mesh Paint Mode has been upgraded. The update preserves the functionality of the original tool, but provides a smoother workflow and UI.

Gameplay and Scripting Updates:

• Improved Convert-to-Blueprint. When creating a Blueprint from multiple selected Actors in the Level Editor, you can now make a Blueprint with a single master Actor that has one Child Actor Component for each of the Actors you selected. This means that you can now save groups of Actors to a single Blueprint and place those Actors around your Level, or in other Levels, by dropping in (or spawning) just one Blueprint.

Mobile Updates:

• New Default Rendering Feature Levels. OpenGL ES3.1 is now the default feature level for Android, and Metal 2.0 is the default feature level for iOS.
• Android NDK 21. Unreal Engine 4.25 now uses NDK 21, providing Android toolchain improvements and optimizations as well as improved compatibility for Android code libraries. This will require a new setup process, as UE4 Android developers must now install Android Studio instead of CodeWorks.
• Android App Bundles. In 4.25, developers now have the option to create an app bundle build for the Google Play Store instead of a traditional APK. Instead of creating a range of APKs for the devices you want to support, the Google Play Store uses a single app bundle (AAB) to create a customized APK for the user's device when a user downloads an app. Additionally, the final APK distributed to a user's device through an app bundle build can have a size of 150 Mb instead of 100.
• iOS Launch Storyboards (Beta). We have added support for using XCode storyboards as launch screens on iOS devices, providing both improved means for scaling iOS launch screens and animation features. Apple now requires the use of storyboards instead of static launch screens.
• Android Game SDK (Beta). We now support Google's Swappy frame pacing solution, providing more stable frame rates and improved touch input latency.
• Eye Adaptation on Mobile (Beta). Auto-exposure can now be enabled on mobile devices alongside other post-processing features.
• Virtual Texturing on Mobile (Experimental). The virtual texturing features introduced in 4.23 are now available for use on mobile devices.
• Added support for Google's Billing Library 2.0.

Networking Updates:

• Networking Insights (Experimental). Unreal Insights now includes Networking Insights to visualize network gameplay data and identify performance bottlenecks or faulty code.

Niagara Updates:

• Audio Data Interface. The Audio Oscilloscope and Audio Spectrum data interface modules give you the tools to create audio visualizations using Niagara in UE4.
• Direct Reads. The Particle Attribute Reader data interface module gives you the ability to directly read parameter data from other parts of the source emitter, or to target other emitters and get parameter data from them.
• Parameter Panel UI Update. The Parameter Panel UI in the Niagara Script Editor has been improved, and now more closely resembles the Parameters Panel in Blueprint Visual Scripting.
• Effect Types. The new Effect Type Asset enables you to set certain common properties that apply to all effects of that type.
• Emitter Inheritance Reparenting. Inheritance is a powerful tool in Niagara; now you can change the parent of an emitter. When the parent is changed, any modules in the child emitter that match the new parent will be updated; but anything that does not match the new parent emitter will be preserved on the child emitter.
• Platform Scalability. There are now ways to directly change how your emitters and systems perform on different platforms. This enables you to improve the performance of Niagara across platforms. Scalability settings are in Emitter Properties as well as in the Effect Type Asset.
• Removal of Namespaces from Module Authoring. The use of Namespaces has been significantly overhauled. When you declare the usage of a parameter, you assign the parameter to a specific category (Inputs, References, Outputs, Locals) and give it a target Scope to read from.
• Scratch Pad. The Scratch Pad is a miniature version of the Script Editor that displays right in the Niagara Emitter and System Editor. With the Scratch Pad you can experiment with creating custom modules and dynamic inputs, and you can either use them locally, save them for use in other emitters or systems, or delete it and start over.
• Selection Stack UI Improvements. We have improved the Selection panel's UI! Now you can cut, copy and paste modules, inputs and renderers.
• Simulation Stages (Experimental). We have a new framework in Niagara called Simulation Stages. This framework has a set module (called a simulation stage) that evaluates all particles before a subsequent stage.

Rendering Updates:

• Auto Exposure improvements. Different metering modes have had their settings unified so that it is easier to work. There are several new features and changes that go along with this update. Also, with these changes means that we have also broken backwards compatibility and provide an upgrade path. However, you will want to take these changes into consideration for your project. Look for our "How auto exposure has changed in 4.25" technical blog for more information.
• Shader Model Improvements for Anisotropic, Clear Coat, and Transparency. Improvements include:
  • Anisotropic Materials (Beta). UE4 now supports anisotropic material properties. It can be enabled in the Project Settings under the Rendering section and use the Anisotropic input on the main material node to control its strength.
  • Clear Coat Improvements. The clear coat BRDF has been improved to be more physically accurate with punctual lights. Use the Clear Coat Shading Model and enable the Dual Normal settings in the Project Settings under the Rendering section.
  • Thin Transparency. We have a new Shading Model for Thin Transparency that enables you to achieve PBR tinted and colored transparent objects, such as tinted glass. Use the Thin Transparency shading model and the Thin Transparency expression in the Material graph to set up and use this type of material.
• Ray Tracing Updates. Includes:
  • Niagara Mesh Emitter support for GPU and CPU
  • Supports the Anisotropic and Clear Coat Shading models
  • The Path Tracer now supports Clear Coat materials for ground truth reference image comparison.
• Hair Rendering and Simulation Improvements (Beta). Improvements include:
  • No longer need to use a Niagara asset with your Groom Assets. These properties are exposed directly to the Groom asset now.
  • Additional options added to the Groom Asset to control the Tip, Root, and Hair Clip length.
  • Improved stability, performance, and additional workflows, especially around load time of assets.
  • Better handling of physics asset interpolation for better collision detection against the body.
  • Groom are now transferable between meshes that share the same UV space.
• Sky Atmosphere Updates. Includes:
  • Transmittance is now evaluated per pixel ensuring luminance on planet surfaces viewed from space looks correct when enabled via console variable.
    • Enabling this also enables the planetary atmospheres to cast a shadow on nearby moons.
  • The SkyAtmosphere component now has a mode to transform it relatively according to the component transform and support hierarchy.
  • Material Layers (Beta). Improvements include:
    • Blueprint functions to modify material layer parameters at runtime.
    • Improved behavior for base materials that makes layer changes to be propagated to any child instances.

Sequencer Updates:

• Adding Spatialize Master Audio to Sequencer. You can now add Spatialized Master Audio to a Sequence by attaching audio sections to individual track components at specified times. Audio icons are drawn at the attachment points in the viewport to help visualize the spatialization.
• Camera Cut Blending. The Camera Cut track now supports blending. This allows you to easily blend in and out of a cinematic or blend between different cinematic cuts.
• Improved Camera Shakes: Spatialized camera shakes are now improved for ease of use and creating more realistic shakes, and eliminates the need to use Blueprints to setup the shakes.
• Take Recorder Enhancements. There are several quality of life enhancements to streamline using Take Recorder and improving the overall user experience, such as specifying the frame rate of a recorded sequence and recording keyframes.
• Template Sequences (Beta). Template Sequences allow you to easily reuse an animation sequence on multiple objects and cinematics, such as dynamically playing cinematics on arbitrary characters and props. They also reduce asset duplication and saves time overall.
• Introducing Movie Render Manager for Exporting Media (Experimental). The new Movie Render Manager focuses on improved quality, ease of use, and streamlined integration into production pipelines, and is perfect for non-real time situations, such as creating game trailers.
• Animating Rigged Characters in Level Editor with Control Rig (Experimental). Using Control Rigs, you can animate rigged characters in the Level Editor, and then export them as animation sequences to use game.

Virtual Production Updates:

• nDisplay Improvements. We have added several features designed to improve the experience of using nDisplay, including:
  • Added support for specifying GPU affinity in order to leverage multiple GPUs and optimize overall system complexity reducing the amount of computers required for nDisplay projects.
  • Implemented API RHI layer interprocess texture sharing to allow sub-processes to access gpu texture data back and forth in a safe and efficient fashion.
  • Implemented Nvidia’s new API for synchronization to avoid special cases of tearing.
  • You can now specify a custom mesh to use to warp the rendered output.
  • You can now specify viewport rendering scale in Blueprint at runtime or parameter in the config.
  • Ability to specify a collection of layers to hide from wall rendering.
  • Ability to specify multiple camera frutums for productions utilizing multiple cameras simultaneously.
  • Ability to rotate the rendered output to accommodate the most efficient wall layout for the physical set.
  • Improved how LiveLink works when used in conjunction with mult-iuser workflows.
  • Improved stability of greenscreen tracking markers using the UVs of the warp mesh or screen space for mapping.
• 3D Text Improvements (Beta). 3D text has received several improvements for this release, including:
  • The Text 3D Actor now supports native True Type Fonts (.ttf) or Open Type fonts (.otf).
  • 3D Text meshes are now cached for better performance.
  • Procedural animations can be applied to 3D Text using the Text3DCharacterTransform component.
  • 3D Text can be extruded and beveled.
  • Materials can be applied to 4 faces: Front, Bevel, Sides, and Back.
  • Multiple formatting options are now available, such as alignment, kerning, line spacing, word spacing, etc.
• Timecode Improvements. We’ve improved how you work with timecodes in your Unreal Engine projects to be more robust and easier to setup and troubleshoot without needing to involve an engineer.

XR Updates:

• Hololens 2. Unreal now supports App Remoting from a desktop app without Unreal Editor running. We also added OpenXR support displaying pixels on HL2, late stage reprojection, enabled mixed reality capture from a 3rd person camera view, and eliminated render target copies for the frame, depth, and 3rd camera render buffers, which should reduce latency by 2 ms/frame.
• Azure Spatial Anchors Support for Hololens 2 (Beta): Azure Spatial Anchor support for the Hololens 2 has been added to Unreal. Spatial anchors will allow holograms to persist in real-world space between sessions.
• Magic Leap. For developers targeting Magic Leap, we added support for setting up shared world experiences using new GameMode, PlayerController and GameState classes, added more utility APIs for content persistence, added API integrations for Connections and camera intrinsics, Revamped Magic Leap's the AugmentedReality interface to make migration of handheld AR project to Magic Leap easier, improved performance and stability of Zero Iteration, added Visual Studio Debugger support for Blueprint-only projects, and provided support to change thread affinities via config vars.
• Motion Controller Keys Removal. Motion Controller keys were deprecated in 4.24 and are now fully removed in 4.25.

Source: https://forums.unrealengine.com/unreal-engine/announcements-and-releases/1728453-unreal-engine-4-25-preview