I sincerely believe that the income for sacking, raiding, trading, and selling prisoners really needs to be buffed. When you get 75 gold for selling prisoners it really just isn't worth it and the same goes for raiding and sacking. The amounts you get is so little and seems out of proportion in comparison. Going on a campaign to raid and sack a few settlements really doesn't pay in the end as your upkeep will far exceed any profit you would make in such an expeditions you are better off to disband your armies and use the upkeep money to build with. These options should be a viable path to gaining a large amount of wealth and is the area that needs the most attention income wise. Trade resources could also be looked at as even in late game with lots of trade buildings upgraded I am just making about 1000-1500 from trade.