Today I'm going to do a little stream-of-consciousness writing on classes and races in my vision of a Proto-Indo-European campaign setting.
Fighters - All are members of the upper tier of society, basically kings, chieftains and their families. Three distinct groups fall under this heading. The Werewolves are an initiatory society made up exclusively of unmarried men and adolescents. They dress up in wolf skins, howl at the full moon and often make nuisances of themselves. Think soccer hooligans with spears. Married warriors are Brothers of the Axe. Most of them don't actually wield battle axes in battle, but the weapon is ceremonially important enough that at least one PIE culture is called the Axe People. How could I not use that? The third group would be an Amazon order. Note that all these groups would be mechanically identical. I just want some additional social dimensions for the fighter types.
Clerics - I could see maybe using the default cleric class if the adventure strayed over to the Nile or the Fertile Crescent, but the local divine casters would all be druids. The cleric is just too citified for the barbaric people I'm imagining. Furthermore, I think the default alignment of the society will be Neutral. Law represents the organization principle of the city dwellers. PIE society would see itself as being organized as a mediating force standing between the fires of Chaos and the ice of Law. Also, druids are cool. The druids represent the middle class in the society. Everyone participates in the religious life of the community, but these folks are specialists in ensuring proper observances and crafting correct ritual objects.
Magic-Users - I mentioned yesterday that since there's no writing, spellbook-based M-Us are replaced with the wizard as epic poet. Herb over at Places to Go had some good thoughts on this subject yesterday. I'd only add two more things. M-Us would be allowed to wear armor and wield spears just like everybody else, though they couldn't cast a spell with a helmet on. My reasoning for this change is simple: when an enemy tribe's host comes over the ridge it's everybody's job to protect the community. Busting mad rhymes does not excuse you from this obligation. The other thing I would point out is that magic-use is a lower class gig. M-Us are all basically the sons and daughters of shepherds. Learning poetry while minding the flock is one of the few sources of upward mobility.
Thieves - These guys are right out. The world is sorely lacking stuff to pick, as both locks and pockets have yet to be invented. Stealing is something that anyone of any class can do.
Demi-humans - These folks would be unlockable content. The only way an elf, dwarf or halfing could join the party would be if the PCs find an enclave of such people and befriend them. Even then, I'd only allow a single token demi-human in the party at a time, or maybe a pair of twins. Befriending the demi-humans won't necessarily be easy, either, as they aren't initially friendly to humanity. Think of it this way: the difference between an elf and a hobgoblin is how well your reaction roll went. Dwarves would be the grumpy little jerkwads from Nordic mythology. If you can bargain with them maybe they'll make you a magic item. Halflings might be homo floresiensis, which would probably end up playing out as cannibalistic mini-sasquatches.