Looks good on paper, but you'd be upset if it actually happened, it would plain suck.
However, if I was going to do it, I'd do the following:
Races: Humans, Predators, and Aliens, obviously.
Predator: Hunter (balanced class)
Brawler (close combat specialised)
Blazer (ranged specialist)
Elder (support class with decreased battle skills due to age.)
Alien: Drone (balanced class)
Runner (speed and agility specialised)
Predalien (average speed, physically tough, weak armour)
Praetorian (heavy armour, good strength, slow)
Human: Marine (balanced class)
Medic (support class with weaker weapons than marine)
Battle synthetic (Combat exclusive, no non-battle abilities)
Sniper (accurate, most powerful ammunition)
I'd have it all take place on one world, but have seperate zones to travel through.
PvP would be pretty much the only goal of this though, as there are no other major races in the AvP universe, so wandering creatures would have to be NPCs from another species.