Looks like the classic alien won't be appearing in Ridley Scott's prequels. He says they are out of date. In an interview with Screen Daily, he reveals that he plans to redesign the aliens.

The film will be really tough, really nasty. It's the dark side of the moon. We are talking about gods and engineers. Engineers of space. And were the aliens designed as a form of biological warfare? Or biology that would actually go in and clean up a planet? It will take place in the years before that, when they first come across this thing on a planet called Zeta Reticuli. ...
Yeah, the thing about Alien vs Predator is, I know it's commerce, but what a pity... I think, therefore, I have to design - or redesign - earlier versions of what these elements are that led to the thing you finally see in Alien, which is the thing that catapults out of the egg, the face-hugger. I don't want to repeat it. The alien in a sense, as a shape, is worn out."

[via NME]