Loyalty, rebellion, and civilian unrest should be a thing, especially when you capture and attempt to incorporate cities from another culture. Settlement loyalty and rebellion seem to appear as a "to be implemented" feature, but when implemented it really should be a big deal and apply this way.
A captured foreign settlement should demand the faction and lord's time and attention, in order to make it stable and prevent riots and revolts, instead of being able to blitz through several cities, incorporate them for your faction, and call it a day.
This would help a lot of snowballing without having to downgrade the AI, and this would also make it more viable to take an enemy city, sack it, get a ton of money, and heavily damage the enemy's economy. The city should take a while to be usable again, and it should kick off in a very poor state when finally operational.
It would be extremely atmospheric, brutal, and awesome to see sacked cities with burnt-out buildings, emitting a lot of smoke, sparsed throughout the campaign map. This game is based on the fall of the Western Roman Empire, after all, sacking and razing cities was happening often and it brought Rome to its knees.