Munin said:
Another idea that has been brewing in my head for some time now is the affect of a truly gigantic metropolis (I'm talking a million+ residents) on a standard fantasy campaign world.
I've done the beginning stages of planning a campaign around a very similar concept, actually.

Didn't mention it earlier because I haven't thought about it in a while, and I'm still hoping to do something with it some day.
In a very brief nutshell, though, the city is (or is at least believed to be) the
only bastion of human society left in the world, and the people's religion has developed to where exile from the city is almost akin to being banished to Hell itself. The city, which has multiple millions of inhabitants, is larger than some states and is still overcrowded. A small tent city of the exiled exists around the city's walls, as people remain close in hopes of avoiding the horrors of the wilderness. The city guards, and the priests who stand on the walls, are assailed by their pleas constantly. In exchange for constant blessings by the priests--blessings that supposedly keep the folks in the tents safe from monsters, by spiritually "extending" the city's protection to them--those outside hunt and cut lumber for the people in the city. A large amount of farmland exists inside the city walls, too, but they'd run out without this outside source. As it is, shortages are all too common.
Most people never leave the section of the city in which they are born, and class distinctions are rigid and (in some instances) brutal. Many social customs have evolved to match the needs of a mostly closed environment. For instance, while tradition demands that dead nobles be buried in finery with jewels and the like, it's an open secret that thieves break into crypts, steal the jewels, and sell them back to the original families who placed them there in the first place.
Damn, I'm getting myself psyched about this again, and I
really don't have time to work on it right now. :\