Here's some ideas I had in another thread:
A)This is for destruction derby modes where you have to be the last one alive.
After your car gets blown up you're on foot and have to look for weapons while avoiding the other cars or other people on foot who have weapons. You can also get a car back by jacking it from someone. Think of how much fun that would be with a 20+ people!
B)How about a mode where there's only 1 car and everybody else is on foot and your in sort of a bull ring. All the people have bats and can try to destroy the car while it's trying to run everyone over. You can't tell me that wouldn't be fun. If you've ever seen those spanish bull pits where everyone is in the ring pissing off the bull and then start running for their lives once it goes after them, same type of deal here only you have bats.
C)Would it be possible to incorporate the gang territories from single player into MTA? You could have the 2 teams blue vs. red/crips vs. bloods. You have to tag a certain wall and that territory is yours unless someone comes up and tags over your work taking the territory back. It'll be vital to protect your gangs tags. Also so the game doesn't go on forever there can be a certain ammount of gang generals that divide up the team into small groups that respawn together(think bf2 squads), only once the general goes down his soldiers can't respawn anymore. Of course there'd be weapons, cars, and anything else that would be helpful in a gang battle(realistically).
D) Drive-by mode- 1 driver, 3 shooters. There are groups of 4 spawn points all around whatever map your playing on along with 1 car for each spawn area. You could make it so once you leave your respective spawn you can't re-enter it so people can't just get out of the car and steal a new one where people didn't spawn. Then of course you drive around trying to blow eachother up.