Dunno if this was already suggested, but perhaps, in extreme unhappiness, your border cities start culture flipping. This kills the strategy of depressed conquests (since you're losing at home while you take abroad), prevents a death spiral by having an automatic way back to happiness (or at least less debilitating unhappiness), and keeps flavor - if a city's that angry, they can join another civ instead.
The first issue that comes to mind is a flip-flopping city between two massively unhappy empires, but I don't think the AI runs in deep unhappiness, so it's not an issue there. It might be weird in multiplayer, but if another player sees a civ's cities flipping culture, it's a sign of weakness and a good invitation to conquer/raze them.
The first issue that comes to mind is a flip-flopping city between two massively unhappy empires, but I don't think the AI runs in deep unhappiness, so it's not an issue there. It might be weird in multiplayer, but if another player sees a civ's cities flipping culture, it's a sign of weakness and a good invitation to conquer/raze them.