I had a dream about this last night so I'm sharing it, don't judge me.
Only available in war, your army would occupy a number of acres of an opposing province. While the army is on the land, whatever buildings are on those acres, the attacking province would now gain those bonuses and the defending province would be out those bonuses.
How long it would last for, I don't know, would it be subject to be% and whose, i unno. Are there other influences to go over, yea.
It's just a thought, a new mechanic for the game developers to maybe play around with and the community to wonder about.