For good Stronghold rules...
Income depends on:
Sources of natural wealth in the area (timber, mines, animals, ivory,....) and quantity of people that exploits those resources. Dragon Mazine 187, 189, 190 and 191 have information about economics and Soldier units costs. Something could be extracted from there. These rules assume a Hexcrawl map, so one would have to follow these assumptions or ignore them completely.....
Other incomes include taxes for commerce, bridge taxes, Door taxes (for getting inside a city)
There should be some way of relating type of terrain (desert, plains, woods, hills, mountains, moors, swamps, coastal and river/lake) to the distribution of those natural resources.
Clima could also be a factor to take into account. It shouldn't be the same living in temperate climate than in a subartical one, for example....
Now... depending on the wealth obtained, the serfs/peasants would have to pay taxes. The DMGR 2 Castle Guide describes a couple taxes for that, but that would be very compliated. it would be possible to unify them but it would be long and difficult.
If you follow BECMI rules, we could also integrate Vassalage and tithes fo the religious orders.
The stronghold rules in the DMG are very light, just a couple building types, their costs and maintenance costs and the neccessary crew for efficiently manning them.
After that, we would need an efficient mass combat rule system for deciding battle outcomes.
Something like the War Machine could be enough, I think....