hey, your idea is promising in my opinion but your realy gonna need a team for this one. I'm making my own mod based just before the Vinilla RTW is set (fictional setting) where you start with very basic troops and can develop more hardy troops as your cities get bigger but everyone has the same troop base so everyone is equal. My mod lacks depth however and i dont expect it to provoke too much interest... your mod however is lending much more depth.

I have thought about the model limit thing - even if we have 9 eras and 19 factions, each one will get 3/4 units per era.
remember you can use the same model for several factions if not all except with different skins, its more the unit limit you should be concerned with (i think its 300) but again you can use the same unit for several similar factions.

Mongol, Chineese and Japanese Peasants can use the same unit definition for example giving you 2 extra units to play with.

I'm a fairly experienced texture artist (4 years just about off and on) so i could lend a hand if you want. You'll have to find some1 to supply me with models tho cus im a terrible moddler :P