I don't know if this has been done before. But I thought it might be fun to make a city that's not in the mortal world, and make building and populating it a group thing. So who do you think would live in this city? What do you think this city would look like? What powers/groups are in this city? Well if I'm starting this I should throw out some ideas.
Places:
The Hill.  Where rich and powerful go to be above it all.
Goblin Mart. A mega mall where anything can be bought and/or sold.
God street. If it's worshiped by some one it has a temple here.
Groups:
The Cops. Keeping the Peace through superior firepower.
Hero's of Hire. The cops can't solve every problem, so hire a hero to take care of your problems.
Biohazard. Magic and mayhem can be messy. But don't worry Biohazard will clean up the mess.