Gang Turf disputes
War for resources

There should be designated blocks in the mix that are tagged as "disputed" and depending on what a gang member does on that block it generates points for their gang to claim that block as their own. Kind of like how you accumulate reputation with a gang by peddling them resources, that pledges and gang members could contribute in another measurable way. The rewards could vary, from something as small as the gang leader giving an automated payout when prompted for their hard work or IC pad rewards, etc.

GTA San Andreas is one way to look at it! I am thinking that these pressure point areas could constantly fuel the combat focused gang members with goals about how to capture and maintain these disputed territories.