one angle that's interesting/fun to play with (or maybe you've already started with this) is looking at how the geography you've created will impact interactions of cultures and then build off of that. I'm thinking things like the difference between a civilization developing in an area with good natural barriers to invasion/attack (mountain ranges, large rivers, impassable desert etc.) vs. ones that have to more actively defend their borders; trade routes and what sort of goods might be moving along them - and what conflict might ensue over control of said trade routes; general access to desireable natural resources giving certain cultures an edge in trade and negotiation (but then also making them a target for invason) etc.
safe to assume that if there is a major trade route, or key natural resources that some civilizations have more of a lock on, there's been conflict over it - and it's safe to bet that the directly involved parties tried to bolster their respective positions by recruiting other allies, maybe even civilizations they had conflict with in the past. or maybe there are a few long-term, enduring alliances. Maybe some civilizations were conquered or effectively wiped out (again: looking at the terrain can help you decide who might have an edge in defense)
also, figuring out who can readily trade with whom can add complexity and depth by the cultural exchange that goes along with it.

anyway, just some somewhat disjointed thoughts - I'm rather sleep-deprived right now.