Allow for purchase of cities and territories.

With that I would add also allow negotiating for building foreign military bases in territories that don't have a city.

Precursor would be having the right foreign policies first. 

I can also imagine some kind of economy victory system that let's you build foreign factories and foreign investments. 

But this is at least the right start in my mind. Makes for more civil and diplomatic ways to get a territory you want without resorting to violence.