Not sure if any of this is possible to mod but I think these would be cool.
1. Able to enslave or loot cities when you capture them like in Total War. When i have like 3 cities already, I dont need them but I dont want to give them to my lazy vassals either, so maybe just pillage them->would be alot of loot and money? and also dishonorable
2. Vassals should be more useful, like paying taxes to king and kingdom from their lands
3. More things to do at towns, high betting games or pit fights where you can bet and watch the fight or maybe join yourself
4. Traits that you get when you do things, like win a 2:1 battle so you get lucky trait or good commander?
5. Forming your own kingsguard, maybe select a leader from your companions or something and special armor and weapons for them
6. Mercenary companies you can hire(alot of men, like 60-200), when you dont have time to train just hire them, very expensive
7. More diplomatic options like in Total War, give village for a big sum of money or offer 20 000 to declare a truce. If they ask for truce, request their castles and money