I thought I'd post this before I forget again...
How about spar bases? Just like regular bases, except every place is a gang spar arena. So, as an example you have Underground, Beach and Complex. Each would be controlled by a spar gang.
Spar Bases would require gangs to hold them based on number of wins per day, in gang spar. This would encourage players to become better at gang spar and increase spar gang numbers.
And, for one on one spar players, there could be a scoreboard ranking system. Yes, I know this has been suggested before, but it would work. People need to be ranked per day! Competition can make these kind of activities thrive!
The top spar player would receive a power up for an hour. Say a free unlimited ammo, or a free weapon repair.
Feel free to throw ideas, questions or negative criticism at me.