There's a Dan O'Bannon interview here in which he talks about his work on Alien, Total Recall, Lifeforce and Return of the Living Dead etc...
In one of the more bizarre bits he does say of Aliens vs Predator film "I did have an idea that they didn’t use, and that was that the alien in his next phase turns into the predator. But they weren’t interested in hearing from me…at least it would have had some continuity between the two ideas."
It seems a very strange notion that anyone would accept that from an Alien eventually evolves a Predator !
Regarding the Alien films he says "It was played out after the first one, as far as I’m concerned. Cameron, in the first one, did about the only thing you could do, which was that he changed to a different genre, from a horror movie to an action film. But once he had done that, there really was nothing left to do. And they just keep squeezing the thing till it’s an empty bag. But as long as it keeps bringing in money to them, they’ll keep doing it."
Anyway click on the link if you want to read more.