Are you trying to suggest that besieging/occupying provinces should have a chance at reducing base tax? That could certainly lead to interesting results...
More like a 5 year negative tax, production and manpower modifier for looted provinces, and 25 year negative modifier for provinces that have suffered occupation. War is hell, and in this game it is just waltzing from victory to victory without real costs involved on the general populace of the provinces.
Economic growth was set back, harvests were looted or destroyed, property destroyed, and resulting in hunger and starvation.
Would give you an actual reason to defend your homeland from being looted or occupied by hostile armies, and give more incentives towards diplomatic expansion.