I remember reading some suggestions for the home planets of the civilizations to feel more unique, and I have come up with an idea that would be relatively easy to implement.
Make the improvement CivilizationCapital different for each of the civilizations, with a unique icon, different stats and have different adjacency bonuses.
For example the Terrans' one could have better bonuses to manufacturing since it was stated in the lore that they discovered fusion power. The Drengin could give a bigger bonus, +2 rather than +1, to nearby military buildings.
If you would capture a civs' capital planet the unique capital would remain and you would get the bonuses for it. While the new capital planet for that civ would have the generic one.
Thoughts? Suggestions?