I think a Conquest mode, à la Battefield, could be nice. It could turn GTA from a racing game/stunting game/kill random people game to a war game
A few positions would be set on the map, all over the map or just in an area of it. Those positions, consisting of a flag or just area and that would be taken over by the presence of a member of an enemy team as no member of the team who owns it is there, would have some "stuff" build around it to kind of barricade it. I don't know what it could be, it could be walls of bags of sand, it could be trucks, it could be and accidented train, a crashed plane, idk.
Around this area, vehicles would spawn, regularly and depending on which teams owns it. Also, there should be weapons and those health and bulletproof vest things.
The spawning would be made at these positions, of course, spawning at a base owned by you. There could also be un-takeoverable spawn points that belong to a precise team, it could be some point in the air from which you would drop, a cargo plane from which you would spawn and jump, or just some point somewhere.
Besides the positions, there would be vehicles to spawn all around, but I think no war vehicle should spawn outside of positions, besides maybe some helicopter.
Still in the Battlefield analogy, there could be a point system, points decresing depending on how many positions your teams hold.
Well basically, call that a Battlefield mod.