The game itself could probably hold 60 players but it's unknown whether the Blue core could hold this amount of players and what the requirements for a 60-player server would be.
I've seen cars with multiple passengers
Maybe this could be achieved in MTA:SA by destroying the passenger and then respawning them when the exit the plane. Or perhaps Blue has an even better method.
Those are good ideas but they're more likely to be third-party add-ons than built-in features of MTA:SA.
I haven't played SA yet but don't you have RPG-like stats relating to fitness and strength etc? If so, someone further in the game is likely to have better stats. Also, I've heard some people say that they've played it on the PS2 and they can't be bothered to go through the single player game again on PC but they'll buy it for the multiplayer.
I think a better way would be to have a set of credits or money when you start a multiplayer game and pick out your skills and appearance beforehand, much like when you start a new single player Deus Ex game.
Respawning at a hospital would be more "GTA-like", but this would just lead to hospital spawn wars instead of robber spawn wars.