I also think 6 cultures are enough for the base game. Although I would love to see some glimpse of other factions. Like having a faction like Western Protectorate of China. Does not really interfere Calradian politics "too much". But sometimes asks tribute, sometimes sends economical aid, sometimes military support. Trade deals etc.
And invading factions would be cool like in Prophecy of Pendor. Multiple sub-saharan clans take a peak sometimes. Nords invade time to time sack and loot and burn and rape and kill and... And having alternate ways to deal with them would suit my play style.
And I believe it could be used to balance the game. Like if Khuzait gets too powerful, Chinese Emperor asks for a tribute. No tribute? China sack eastern settlements.
Anyway, we will see a lot of cool mods. I cannot wait!