So are you wanting to do it based on criminal activity of a "Family/Gang"? So more crimes that gang does in the area the more control they will have and own the area? Sort of like a percentage?
Lets say London has no gangs that own it, so its a "Open Contested Location". From that a gang can start doing crimes there and gain a "Crime Rate". After a certain time the gang with highest "Crime Rate" would win the location:
London:
Gang 1 = 75 % Crime rate
Gang 2 = 15 % Crime Rate
Gang 3 = 10 % Crime Rate
Gang 1 owns London
Then if a another gang wants to take over London from Gang 1, do a battle system:
Contested area's:
So just say Gang 1 owns London, but Gang 2 wants to take it over, so they invoke a "Gang War" against Gang 1. Then players from both Gang 1 and 2 would have to "Join" the gang war before a set time (before a battle starts, 24 hours). Once set time hits everyone who entered the Gang War from Gang 1 and 2 would then fight (Calculate Attack and Def), gang that won would then own that location, in this case London.
Then have a cool-down for a certain amount of days before another Gang war can be invoked at that location.
Doing a type of Gang owned Location, it would be good for a bonus to be applied to the owning Gang members to make it worth owning the location, such as a percentage on top activities (crimes, Theft, ect) done for owning gang members. With this you could make certain locations be worth more and gain better bonuses (London 5% bonus, Rome 7% bonus, ect).
Also set a max number of places a gang can own, and maybe have that up-gradable in the gang menu? (gang 1 is level 2, so can own a max of 2 locations)