I've always thought that the best way to end the original "Alien" film was to have Ripley dispatch the creature out the airlock as in the original.
Then while preparing for cold sleep she opens a storage locker only to come face to face with a mature egg, the last shot would be the face hugger leaping at her!
P.S. Scientifically speaking, how the hell does some thing grow from a the size of a face hugger to a fully mature alien creature, all seven of feet tall, in a matter of mere hours, with no food intake, the material and energy must come from somewhere, including the air that we breath we literally are what we eat!!!
P.P.S. Crossovers do sound like fun, look forward to Batman, Judge Dredd or Sherlock Holmes, how about a Star Trek one, would love to see Data wrestling with it lol!