Uplift Series (by David Brin)

[Batman]

It might be interesting to let some players be the 'Old Ones' (ok, I can't remember what they were actually called) and let them be Ice players on Ringworlds, or Sphereworlds, but deny them the ability to colonize any other worlds at allexcept maybe other Sphereworlds (i.e. put no Ice planets in the quadrant). Then you'd have three levels of conflict in the game:

Oxygen/carbon based, on the rock planets.
Hydrogen based on the Gas giants.
Ancient life on the Ice worlds.

(Note that the atmospheres aren't important; I refer to them in the Uplift context, not the game context)

This would need to be an artificially created map.

Ooh, just had an idea: Include a bunch of neutral races, deny them the ability to build ships (somehow) and leave them around as 'client' races for the players to 'Uplift'