AID
Change how aid and tax is calculated. Make it based on kingdom networth instead of province networth. This will allow kingdoms to help out their small provs more with aid, but the big provs wouldn't be able to get as much. Currently its the complete opposite.
Also, remove gangbang protection as it applies to tax. I dont like how its used by organized kingdoms to create cows. In the current setup, a kingdom will gather a bunch of soldeirs and gold, then ask another friendly kingdom to hit a particular provinces until that province has gbp. While under gbp that province can receive an unlimited amount of aid and use it to grow. Its an unfair advantage to kingdoms which have friends.