I actually think they should just adjust how much hate you get for CS kills. Treat them sort of like they're all part of one larger civ. For the purposes of diplomatic victory, they pretty much are.
Like I said above, it'll make Diplomatic Victory less of a snooze-fest: Either you're going to have to be a knight in shining armor and save those city-states, or you're actually going to have to be popular and convince other Civs that you ought to win.
Like I said above, it'll make Diplomatic Victory less of a snooze-fest: Either you're going to have to be a knight in shining armor and save those city-states, or you're actually going to have to be popular and convince other Civs that you ought to win.