So neolithic is a setting detail, but what's the game going to be about? It'd D&D, so butt kicking for goodness in a high fantasy setting is a given, but what sets this neolithic world apart from Greyhawk, the Forgotten Realms, or even Dark Sun? What's our hook? Once we have our hook we can figure out the best way to adapt the species, background, and classes to the game.
So what's the hook? What makes this different from other settings? In most D&D settings, it seems to me like you're picking over the bones of long abandoned, ancient cultural sites (dungeons). But in this neolithic setting, everything is new. There are no dungeons (okay, maybe a few ancient Yuan-ti structures or something) and maybe a lot of the staples we've grown accustomed to having just aren't there or are only just appearing. Undead? Never heard of 'em. But Uncle Roger seems to be doing now even after we buried him last week.