I'd love to see a colonization & rebellion system be introduced to the game, to balance the maintaining of large empires.
- Cities that you conquer initially would continue to produce culture for whoever had them before. Ex: Paris would continue to make French culture even if Germany had it. The longer you have a city the more it's culture becomes your own, especially if the population born under your reign exceeds that of the population born under its previous owner. This would make warmongering to take advantage of someone else's culture less OP.
This is how it was when the culture feature was first brought in in Civ 3 and the Paris analogy is exactly when they used when selling it! Each city had a breakdown of nationalities that were essentially 100% the original culture and then slowly change the longer you held them. Going to war with the mother country down the road would create unhappiness in cities that still had an amount of original people there.
Was great and another lost feature that they just threw out for no apparent reason and should come back.