I think one of the coolest ways to have it set up would be by hideouts. Say like, Ocean Docks, Mad Dogg's mansion, 4 Dragons Casino. Just for ease of example, we'll use three gangs and one hideout. Gang picks a set of vehicles (I would set limits, nothing too over the top. Like all patriots or something) Possibly a heli, and a boat outback depending on location.
Basically, you can choose to chase after gangs and what-not. Maybe communicate and make a meeting point at an abandon location in the desert. Or maybe. It'd be like, SFM chases after The Bayside Mafia, and is running all over them. Maybe there would be the need for a retreat, gang wars ending only when all members are dead.
Just to base it on pure tacticals, maybe have a set number of "lives" or have usable healthpacks and an extra armor. IMO gangwars should be based for on tactics and being smart than kills and randomly attacking eachother with no real set plan.