Ranks would be a fascinating aspect.
If you wanted to go really in-depth with the gang system and look into really upgrading it, you could try something like this, maybe?
Owner (Handles all aspects of the gang including finances/ownership/recruitment/meetings/upgrades/daily boosts, etc. Maybe players get a reward if they're from a rival gang and they kill him?)
-
Right Hand (Manages all aspects of the gang other than ownership, can recruit new members, bigger HP/Armour boost, special outfit/weapon maybe, can lead power struggles against other gangs.)
-
Underboss (Manages other ranks below him, can recruit new members, slight HP/Armour boost, maybe special weaponry)
-
Enforcer (Sends muscle on tasks, leads them into fights, maybe slight health boost?)
-
Muscle (Proper member of the gang, entitled to gang perks, works under every rank, trains the initiates.)
-
Initiate (New gang members, no real power, not entitled to the gang perks.)
Just spitballing here.