In my case one thing that I'd really love to be added would be an option to convert other civilizations into vassal states. Right now, one can either conquer a planet militarily, by influencing it and (rarely) buy it. Another related situation ocurrs when another civilization, Major or Minor surrenders to you and thus you get all the planets "overnight". This, I think, is too simple- maybe I don't even want those two crumby planets in some distant system, in a galaxy far far away... So why shouldn't one get some option that pop up once any one of the above situations sets in?
"Make the newly gained planet(s) or system(s):
- your colony" (as was so far)
- a vasall state" (i.e. pays taxes to you and assist in war, turns influence points over to you, but keeps 'home rule'- may fall off via independence movement but yields trade/tax bonus over a colony)
- an empty world" (population culled & you move on without conquering, surviving troops back in their transports (UI may ostracise+gain "evilness")
- a free power" (makes freed planet a full fletched Minor power unatached to you or anybody*