I would say that it will be a few stages of a boss. By this i mean level bosses and what not. For example,
1) 1v1 in a power loader (giving the nostalgia final scene of aliens feeling for fans and 'awesome' for people who don't know much) is more than likely going to happen at some point.
2) possibly a no weapons hand to hand with a drone (God of war style combat where you hit circle at a particular time)
3) You play as an alien? (Big time maybe, only god knows about the multiplayer for that as well)
4) standard battle against troops (rumor of the black ops yet again) and you have to fight the guy in charge (Again, 'awesome' or people who dont know much and 'meh' for fans)
5) Flying away, fighting a flying alien

(That would just be awesome flying the plane against a flying alien, imagine all those missiles fukn BOOM!!!!)
6)If they truly run out of ideas, fighting a queen with every gun in the game.
7) Like previously stated, the space jockey. Not much is known, but for anyone that read the comics & Novels (Only comics and novels i ever read were the aliens ones, i generally dun read that much fictional stuff but this is by far my fav movie and franchise of all time) you all know the back story of the space jockey vaguely (Also with the coming of prometheus they might, you know, get some ideas from the film and apply it into the game, showing some stuff that will appear in the film and make people go 'ahhhh ok i get that now')
Guess we'll have to wait and see ay??