I don't think ruling a province should be all cakes and ale, though....
as commented by others, extremely high taxes should have a negative
effect. Perhaps local population might slowly emigrate to adjacent
provinces if they're feeling oppressed. On the other hand, though,
population should naturally immigrate into cities and towns, especially
those with markets.
As a random thought, perhaps provinces should have several variables
in addition to taxation.... a TAX rate, a LAW enforcement rate, a
DISPLAY, and a measure of DISSENT (or perhaps the reverse, a LOYALTY
like units have). TAX would range from 0 to 10, a measure of how much
cash you're trying to rake, and LAW would range from 0 to 10, varying
from lawless to authoritarian. The amount the province lord collects
in taxation would be based on the min of TAX and LAW (without law
enforcement, most people dodge taxes), and might be reduced a bit if
DISSENT is high. Taxing a province heavily would cause the DISSENT
rating to automatically go up. Any combat occurring in the province
would also automatically cause DISSENT to go up (i.e., the peasants
pay taxes; they expect the lord to keep the peace). Variants on the
PERSUADE command could be formed, an INCITE command which causes the
unit to persuade DISSENT to go up, and a PACIFY command which causes
DISSENT to go down. Heck, I suggest that a new skill be put in,
'persuasion', which has recruit, persuasion, incite and pacify all as
subskill uses.
If we want to get really complex, we could also add two new attack
modes: attack revolt, and attack repress. Any unit or stack in a
ruled province could attack <government structure> revolt, and their
stack would get augmented by an additional unit comprised of rebellious
peasants, the number being proportional to the DISSENT rating. The
rebellion attempts to oust the present lord. After a rebellion,
regardless of success, DISSENT would go down. (Either the rebels are now
in power or else they're killed off.) (Or alternatively 'rebels' could
become a depletable commodity in a province, like wild horses, and
'rebels' are regenerated proportional to DISSENT.) A lord or other unit
could attack repress, which would deplete the number of rebels and
raise DISSENT. Rebels killed off should be removed from the province
population, though, so killing off all your taxpayers eventually becomes
counterproductive.
For extremely low DISSENT ratings (peasants loyal to their lord), attack
revolt should cause the peasants to join their lord rather than the
rebels. Or perhaps occasionally peasants would join both sides (civil
war). Or, alternatively, a province lord could have a special command,
CONSCRIPT, which works like IMPRESS but is more effective (unit loyalty
based on the amount of DISSENT).
LAW enforcement would probably have to require some sort of mechanism for
a city guard to be appointed which automatically attacks to capture anyone
acting rather shady (failed stealth uses, entertainment without a license,
or, for very high LAW ratings, just being a stranger in town). If LAW is
too low, random lawless events should occur; bandits might raid peasants
and disrupt tax collection, for example. If LAW is too high, perhaps this
too would raise DISSENT.
I guess that's enough incoherent babbling for now...