Would you consider doing a Flashpoint: Fallujah?
This game mode on an open map can be frustrating because it can be hard to survive against full enemy squads when you are plopped down in the open. But the high density of enemies and random placement would be brilliant in a large urban environment.
It would be necessary to restrict spawn points to the city proper, but endless fighting in a maze of cover and corners would be incredible.
Perhaps simply release a mission with the script active so people could fire up the editor and place themselves down somewhere to fight waves of infantry? Then there would be no complicated conversion and no worrying about players landing on rooftops.