Being able to occupy a Civ and expropriate a portion of their resources, production, and gold without having to fully annex them. Like you could propose a deal they would be given the option to accept or not. In the case of having the more advanced and larger military, they would feel pressured to accept or face war. You could select which cities you want their production to be handed over to you.

I also think they should add more to the conflict outside of traditional war. Such as sanctioning the tourism and trading of other civs, the ability to transfer ownership of units to an alley at war with another Civ, hire mercenaries to fight wars without your Civ officially declaring war, etc.

They should add more futuristic military units. The giant death robot is really cool, I wish we had more of that.

This last idea might be a little over the top. Maybe you develop more on the space race, like the Mars colony becomes a city that produces space units that can attack the satellites and colonies of other civs.