automatic donations would make no substantial difference compared to ordinary gold transfers except they would, yes, allow lords to be more passive.
my idea mostly rotates about current income, that is contribution from peasanats which does not lower region tax income at all, but present separate money inflow already.
before upkeep feature it was far too easy to earn money in religions without any effort. right now i feel it tedious to permanently fill in negative net balance which occurs in majority of regions. if upkeep would, for instance be reduced to one third of the current, mid-size religions could possibly earn 200-250 gold per month, which could not be so big deal, but would provide some fun to religious people.
moreover, if priests would not preach enough, income would fall to negative, so main purpose of upkeep would be intact - that would both punish too passive religious game, and would slightly, just slightly reward active ones.
after two-three months of savings, elders would post large bounty for duke who is very hostile to their faith, and with new feature, assassination would have some chance to remove him. that would be the game, without any revolutionary changes.
net income would be small and would just give some sort of independence to religions, and even that only if priests are preaching enough.
