Hey i had an idea quite like this some time ago, if you you like it, you are free to use it
"i think everyone saw that the AI can go a bit crazy, and can expand in a very unrealistic way (and the player can too).
-So if it is even possible, the ability to peacefully occupy/Raze/Loot should be removed for most factions, except: Rome, Carthage, Egypt, Macedon, Seleucids (the remnants of Alexander's empire) or no exceptions if its easier to implement that way.
-The other factions could only sack settlements, take the money, and then move on, or liberate. -- HOWEVER if they DO have their culture dominant in the region, they could take a settlement.
It could be even tied further to the culture:
-if at least 40% of the attacking factions culture is present in the region: they could raze the defeated settlement
-if at least 60% of the attacking factions culture is present in the region: they could loot the defeated settlement
-if at least 80% of the attacking factions culture is present in the region: they could peacefully occupy the defeated settlement
This way, there would be eventually a bigger empire in each culture, or they would fight aganist other cultures and form confederations."
Cheers