After two PC games where you could control Aliens, leaving them out entirely would make the game feel incomplete. I don't mind them skipping Alien control in single player but I'd like it if they included them in some multiplayer modes.
As much as I'd prefer having AvP3, they'd be able to focus more on Aliens and humans without the Predators to worry about. This makes it easier for them to give more equipment/abilities/weapons/senses to Aliens and marines and for them to make sure they are balanced. Overall, I'd expect a sense of balance, yet realism in Alien vs human encounters that would blow away the previous games. Afterall, that's just about what having an Alien game offers that other FPS don't - Aliens.