As a spin off to [@ScreenAcne]'s idea, why not have it be like a cluster of virgin, gaia planets/systems, prime space real state, that we're all fighting over? Some of us could be in it just to colonize and expand, some just see the war as excellent practice for a highly martial culture, and for some, its literally their last hope for survival as they have no where else to go and their homeworld is destroyed or something.

In this way, we can have the mix of races/empires without having to be overly advanced but still have the colonization and all of the politics and warring involved!