That would be very cool if the rival gang leaders were girls (perhaps with their own special type, differentiating them from slave/normal/catacombs girls) who you could recruit/enslave when they are defeated (as alternatives to leading them out of the city/throwing them to the streets/killing them). By recruiting them, you could gain their assets (buildings, money, gangs, whores) as well as having the leader working for you.
What if this was the default way to gain new property? Assume there are a fixed number of buildings suitable for your criminal enterprises in the city, you own one (or two or three, some small number), and the way to gain more is to defeat your rivals and capture their territory and assets. The more territory you control, the more power you gain: you get more brothels, girls, gangs, money, businesses to extort, etc.
Maybe divide the map up into sectors, with each sector having some resources for the owner to draw upon (buildings to own, some number of potential customers, gangs to be recruited, special buildings and resources - a catacombs entrance, a viras plant farm, a hospital, etc).
Maybe I'm getting carried away here too >;o)