Okay, I'm not expecting it or anything, but just imagine, similar to Firefight Versus, 4 are selected as the humans, battling through campaign, and 4 are on a team, spawning along the way (similar to in L4D, where they are in a "ghost mode" and they find a spot to spawn as a type of Elite (multiple loadouts) that has to be out of sight/certain distance from the humans. Imagine even farther... You start out with only a couple of Elite loadouts. For how much damage you do/how much you impede the human's progress, you earn "in-this-match" points, which can be "spent" on better loadouts/other classes. Once you spend it though, it's spent, and you only spend the next life as that class. If you spend it and die immediately, you won't have any good loadouts next round. Like a Jackal sniper, better Elite loadouts, Brutes, maybe even spawning in vehicles depending on the level, Hunters, etc.
the amazing game, Perfect Dark, on the N64, had a campaign versus type mode called Counter Agent, if i remember correctly it spawned the "counter agent" at the end of the level and the agent at the beginning... i dont remember exactly how the objectives worked and stuff but i just remember the counter-agent worked to stop the player, and it was awesome.
Grunt special ability. Suicide Jackal special ability. Regenerate shield Elite special ability. Evade Lol i made thread bout this ages ago