Re: Why do the aliens want Ripley?
I don't think that the Aliens want Ripley at all. I doubt that they'll even know that she exists. She's a plot device for that i central to the stories. Aliens would have been a rubbish film if Ripley woke up 57 years later, but missed all the action.
That said, i remember reading the old Aliens Dark Horse comic where the alien gets to and infests earth. I'd LOVE to see that as a film.