[Idea] Simulate Corruption and Inefficiency
Just throwing this out there for possible inclusion.
As city influence expands, the number of potential resources increase exponentially and it feels a little too powerful. How about simulating something like trade route inefficiency/petty theft/whatever by reducing output based on distance from the related city. Further to that, how about simulating corruption by applying income variance based on distance from the captial city?
There could of course also be more researchable techs to supplement this system.
EDIT: While not directly related to the subject i just had another idea about resource availability. Make resource nodes of variable quality. This could potentially encourage "shopping around" rather than just grabbing every resource you can find, especially if combined with the maintenance costs GoaGalneGbilski mentions below.