The liberation game can be something to do later on. It's pretty clear that there need to be more diplomatic options for city states though. Your options for keeping city states later in the game should not be: A: dedicate 6 units per city state to surrounding them at all times B: don't make alliances or DoFs and be ready to declare war even on your closest trading partners. There desperately needs to be some kind of diplomatic "C: stay the hell away from my city state" option. If they are allied/dof with you and ignore your warning you can DoW them, and with no penalty.