Suggested by Shadow, slightly adapted by me.
So there's a page on 'city' where you (as an individual or gang if you're leader) can claim cities as yours. If they've been claimed and are in control by someone else you have the option to claim another city or challenge them. If you challenge them you/you and your gang hit the current city owner until they surrender and step down thus leaving the city to be claimed.
If the city you want hasn't been claimed you can claim it. You/X amount of members need to stay in the city to keep it claimed. When people enter you can charge them for anything or threaten them with hits while they are staying in your city. If they use the gym/hit someone in your city the leader of the gang gets an event saying who, what and when. Then you can hit or charge for it.
Some cities (Phoenix and Salem for example) are controlled by the police and cannot be claimed. You can stay/train do whatever you want in these cities for free.
Thoughts?