regular writing with a special font and perhaps all lowercase sentences

yes, that's what it has right now.

I'd maybe look at how much vocabulary Coco (the Guerrilla who was taught to speak sign-language) had

from what i recall of that and work done with symbolic languages with both chimps and birds, sentence complexity is not really high - perhaps 6 symbols max, most are on the order of caveman-speak. "coco - want - apple - question?" that sort of thing. vocabulary size wasn't bad though in the hundreds for coco i think. it would be a place to start.

if you apply it consistently

this might be the big issue right here. all existing text (or at least dialog) in the game would have to be changed to caveman speak to apply it consistently.

The problem with caveman speak is that it takes much longer than actual speech, sounds stupid, nobody ever actually talked like that and the people it's applied to actually were not at all stupid so it doesn't make any sense. It just doesn't work, even as comedy.

The problem with caveman speak is that it ... sounds stupid, nobody ever actually talked like that and the people it's applied to actually were not at all stupid so it doesn't make any sense. It just doesn't work, even as comedy.

Its true that our modern projection is tantamount to prehistoric black-face, but its certainly true that prehistoric man had a lesser capacity for linguistics, lacking both physically and mentally, and were unable to engage in complex language. To the extent that they were able to engage in conversations, they would have used grunts, yelps, and other sounds to communicate, and whatever recognizable vocabulary they'd have had would have been limited to the things they were concerned with in their immediate proximity and observed first-hand (because you *need* complex language to communicate far-away/unseen things). If we had recordings of this, the literal translation might not sound that far off from what caveman speak we've invented on our own.

but its certainly true that prehistoric man had a lesser capacity for linguistics, lacking both physically and mentally,

right now, caveman only features homo-sapiens. earlier species (cro-magun, habilis, erectus, neandertal, etc) would simply be more limited gameplay options. i may add them as extra challenging play options. but homo-sapiens is us. so all the physical and mental capabilities are fully formed. as you say, the preceding species would be more limited in those respects.

back in 2000, i created a caveman alphabet for the original game. 4 vowels: ah, ee, oo, uh. and 4 consonants: B, G, M, and Y. these yield 20 syllables total (including single vowel syllables). i wrote a random word generator routine. enter the number of syllables, and it generates a random caveman word. its on the tools menu for the game. its also used to generate the names of NPCs. i also used it to come up with caveman names for the animals in the game. almost all the animals have a scientific name, a common name, and a caveman name. in the original version, they were all made-up animals with caveman names.