As of right now there appears to be no reason whatsoever to waste time with city states, since the AI players just gobble them up and there seems to be nothing you can do about it whatsoever other than declare a surprise war to defend them. I'm shocked there's still not a "you're at war with my city state" CB, but I guess it's a "surprise" war because the AI is "surprised" that anyone in the game would give a crap about city states. I imagine this is basically impossible to mod, since it would require a complete change of the way diplomacy works in the game and to teach the AI that attacking its allies' city states is a bad thing (which it does not understand right now because there is quite literally no downside to attacking your ally's city states). I figured I'd ask anyway, though, since I'm not as up to date on what mods can do in 6 as I was with 5. Oh, a mod that lets city states capture cities would be pretty cool too, so I imagine that's probably also impossible, as everything I want modded about this game is unmodable.