The answer to all your problems is running through my mind. It's easy, just creat one HUGE! level, of the entire city of Mexico City, add 3 vehicles (humvee, raft w/ gun, black hawk), write software that creates random spawn points X amount of meters from enemies, allow for 100+ players...have fun!
Seriously though, watch that ghost recon 3 video where the 4-6 guys blow up the tank, & 1 gets hit. When they look down the street it seems to go on for miles & miles, it would be so awesome, just sneaking around through a vast city, and be able to go into every building and take position in a high window...that would be pretty cool!
Edit: Of course the raft may not be included, I'm not sure if there is any water in mexico city