Just came up with this after watching 8 mile. Im not sure whether there is a mod like this already, I no theres crime city, the GTA mod and the EC mod.
But you cud start of in a gang and each time u complete a mission u get more street credit and more ppl are willing to help you out. Main weaponry wud b pistols, ingrams, and possible AK47s. The police of course wud have heavier weaponry. U'd need new skins and vehicles but it would be cool, and you wud get choices to like betray your best friend for your own benefits or stay loyal and take the heat with him or somethin like that.