Loads of things.
If I went to an inn and bought it , I could affect the economy by changing inn prices.
Or if I destroyed a farm by catching everything on fire or poisoning the farmer , people would starve to death , and so I would add or subtract a variable from the population of X town.
It's really amazing what I could do with banking and such.
In my opinion , banks are the most simple , yet can be the most complex , considering that everything revolves around money and the needs of the people.
It is my understanding that a system like the one you describe is, while probably innovative
and pretty interesting, something that would devour a lot of development and programming
time. If your game doesn't mainly revolve around this system there's a good probability storytelling,
character development and additional gameplay will lose precious development time and thus its
place in the game.
I believe that an 'advanced idea' can be a quite simple implementation if it offers the player a new
or different experience. Something that can keep people interested.
I, for one, am very interested in games (even linear games) in which quests or storylines can
be played differently, resulting in different outcomes, different treasure or items, different skills
or even different quests.
Let it be said that what I suggest is not an easy implementation, merely an advanced idea that has
more to do with storyline than systems.