[sachmo]

Well, there is another possibility...

I don't know how many are familiar with the Battletech story, but you could do something sort of based on the Clans.

You would have one of the splinter Groups, preferably based very far away from the others, but with pretty advanced tech starting off. In the core systems, all of the splintered Groups would be fighting for survival/ascendancy, and basically pummeling each other into submission. Eventually, the advanced splinter group decides that THEY are the ones who will recreate the grand republic, and they come looking to subjigate everyone...
I always loved that story...might be fun to play!