basically something like, you can invest money into cities, harvest materials for the city and eventually it would grow, from a small city getting larger and larger, including new things, like guild halls, shops, houses to buy etc - The list goes on

this could also incorperate new skills to harvest materials.
You could also fet rewards, like discounts at shops, bigger houses to buy etc.
It could also have things like, helping to manage the city itself, having benefits and consequences. For example, you could increase wages to certain workers and in return you would recieve more of what they harvest and they work faster altough you have a certain amount of money you can give, so you would have to decrease wages in other areas - consequenting in less working being done or people quitting.
You could also manage trading with other cities to get supplies, the more you buy off a city - the better relationship you have with them and they could offer other services. But it doesn't just have to be limited to materials, it can be soldiers etc.
I got this idea from a minecraft mod that allowed you to harvest materials for a city and then they hire builders to use the materials to build things like shops, farms etc.
if anyone likes this and has anything to add to this idea i'll add it to this post

EDIT: http://www.gamesas.com/index.php?/topic/1273488-req-i-own-a-store/ <- a BRILLIANT idea that would work wonderfully with this one

