So some ideas that I have are:

Time: Modern day or 1930's
We can all be in a gang together with the police trying to shut us down, or we can have rival gangs, or rival gangs with the involvement of the police. 
There will defiantly need to be a gang leader if we decide to do one gang, and at least one gang leader per gang if we decide to do two gangs. 

Just some ideas that I wanted to throw out there. If you have any, please share!! :D