ScottDoom
Recruit

Although I think the entire game wouldn't be suited for multiplayer action, I think it would be great if you could have a mini-game add on where maybe 10 human players could fight it out in a huge custom-designed battlefield either on teams or in a free-for-all. In other words it wouldn't be part of the main game, but another option in the game menu. And maybe you could select from a few pre-made characters for multiplayer combat or import one of your own.
Of course, I have no idea how hard the netcode for a combat system like this would be.
Of course, I have no idea how hard the netcode for a combat system like this would be.