1)
I think it would be nice to differentiate races by varying their access to extreme planets in their tech trees.
For example Yor could start out being able to colonize barren planets while colonizing aquatic planets would be an endgame tech; whereas for the Torians it would be the other way around. Maybe some races don't have techs for certain planets and they have to trade for the tech or gain it otherwise (espionage or conquering the planets -- if this ever becomes an option).
2)
I think it would also be nice to have a game mechanic that allows to have multiple races within your empire, e.g. through tourism or conquest or other means (refugees) your planets could have mixed populations, say 10% human 80% talan and so on. Your type of empire/ideology would then control how those foreign elements are handled, e.g. if you have a benevolent republic they are just citizens, if you have an evil empire they are cheap labor, but prone to revolts especially they are prone to switch sides via culture flips or fight an the other during invasions. some races could be pure genocidal, e.g. all Yor planets are always 100% Yor. This could add to a lot of depth to the diplomatic and ideology gameplay systems.