One game mechanic that might be interesting is an AI controlling all the other gangs and having them battle amongst themselves for territory. Let's say there are four gangs: A,B, C, and D. You're friendly with A, hostile with B and don't care about C or D. You go into a neighborhood now and A is in control, so they are cool with you, give you help, you can buy guns or get info or something. A bit later the same neighborhood might be under control of B and you have to watch you back. Later on you go to the same neighborhood and it is actively a warzone as B, C, and D are all fighting for it. You can aid a gang but it affects your reputation with other gangs.