|Possible gang illustration|
First, though, we need an endgame. At what point does the group effectively become the dominant gang in the city? I'm presuming that means a certain point of manpower, a number of gang members under their control. But how many? And how does the gaming group get them?
From there, I think there should be some kind of both recruiting and maintenance cost for each person: a certain amount to hire and equip, and a certain amount to retain. The hiring cost should be a little higher, and make attrition an issue (it's cheaper to keep hirelings than replace them).
Then, there should be some rules for what the gang accomplishes each time period (week, month, whatever). The gang is theoretically doing gang stuff: providing protection to the territory, making forays into other gangs' territory, generating income, etc. In that there is the potential for losses, both financial and membership attrition from injury, death, and desertion. There should also be the chance for recruiting new members.
Compounding those rules should be ways in which the gang's management, the PC's, can positively or negatively affect the outcome. Wizards should be able to provide magical support, clerics heal wounded members, fighters train people, and rogues set up deals, etc. This should both be an opportunity for for providing assistance, but also have risks as well. Maybe the wizard's magical help backfires, or the rogue's information is actually a trap planted by another gang.
So I'm thinking I've got some actual rules creation to start crunching on. But one question: am I reinventing the wheel? Are there rules in Adventurer Conqueror King or something similar already?