I don't have humankind yet so i don't know if these ideas are already in the game, but it will be nice to have more diplomacy like you could start unions, coallitions, treaties with other empires. Treaties could be about everything from the decision of which empire gets which part of a previously conquered empire to the decision on makin a new empire/state. Another idea is to have the possibilty of your cities to revolt when it has a different culture, is surrounded by other cities, if it has insufficient food, a spy revolted the city or it's isolated/ far from any other city. And maybe when a city revolts it will be for some turns in anarchy and then would it possibly be it's own empire or wanting to join an other empire. It would also be realistic to have some diseases that could start which could infect food and cause a famine or start to kill people directly.