I personally loved the having races be individual units with unique characteristics in MOO II, I'm Sad Galciv didn't have something similar, because in MOO, when you conquered another civilization suddenly you had an entire new race with new characteristics and you had to decide what to do with them: Enslave them, integrate them, or exterminate them.
I personally conquered races who had bonuses my race didn't have: soon my warrior race had soft squishy scientists doing all the research and hard working industrialists doing all the industry, with my own warrior race guys proud to keep doing the war.
I didn't know that was in master of magic, too, but it looks like it was a similar system and I always thought it was a cool way to remind you you really were conquering different races and they didn't just vanish and become you when you beat them.
I think having something like that in Elemental, if you end up having different races, would be cool.
Different cultures of the same race wouldn't quite work since they can interbreed, but it was cool to have an empire with castes.
But then, maybe it's something for Galciv III? 