Instead of (or as well as?) trying to sort out a system for PC gangs, maybe it would be cool / beneficial to make being involved in the NPC gangs a bit more appealing?
I am still pretty new to Sunberth, so I may be a little off-base on this, but there is a little bit of a sense that the NPC gangs are a bit out of sight / out of mind. Unless you're directly affiliated with a particular gang, there's not many reasons to go into gang-controlled territory; most of what people need is in the unclaimed areas. While the gangs do show up in those areas (the incident in the Seaside Market at the start of this season for example), it's possible to avoid gang involvement almost entirely, if you're even a tiny bit careful about it.
It might be interesting to shake things up a little bit. Perhaps (now that the Scars are gone?) the gangs have started trying to get control of little pieces of those unclaimed territories. Perhaps certain taverns, businesses, etc are the victims of racketeering - maybe the Drunken Fish has been forced into loyalty with Gang A, while the Pig's Foot is now loyal to Gang B; perhaps a particular gang has taken over the Market, and/or a new NPC gang has risen to "protect" the market from the other gangs, but is charging protection money from the merchants to do it. The Knight's Armory already provides a discount to the Sun's Berth; perhaps Whiteflame's Cider Mill has been forced into loyalty with a particular gang, which means that the taverns affiliated with that gang are able to offer cheaper cider than elsewhere. Maybe over in Baroque Bay, different gangs (or one gang?) has taken control of certain piers, which gives a particular gang control of certain types of item coming into the city; perhaps a different gang has taken control of the storage houses, and there's some mafia-style deals between certain gangs - Gang A can bring in textiles from Sahova, but they need a deal with Gang B to store it in their warehouses, until they're able to sell it to the merchants, who have to pay a cut to their Gang C protectors... some sort of system that makes the gangs and their organized crime antics a bigger part of every-day life.
Also, it might be cool to see some sort of perk system based around the kind of standing people have with those NPC gangs. If there was a system that allowed PCs to advance within the NPC gangs, there might not be as much of a need for PC gangs - being the leader of a small group might be less fulfilling than being a mid-level guy in a larger group, with both PC and NPC underlings to make your life more interesting.
That's just my preference/perspective though, so it might not appeal to anyone but me.
Also, sorry for being so long-winded.