By "Alien" I meant the franchise, rather than just the first film, and the later films exist now for good or ill. But yes, I think another film would probably not involve Ripley, and could be set between the second and third films. I'd like it to use the species that was piloting the ship they found in the first movie, which was not human, Alien or Predator. That new species hints at a wealth of possibilities. As does the idea that the Aliens were originally genetically engineered as weapons of war. Who was fighting who? Predators versus Pilots? Is that war still going on, and if so, where would humans stand?
Switching movies now... the commentary for Run, Lola, Run mentions a possible sequel covering what the tramp did with the gun. I don't think R,L,R really neeRAB a sequel, but as it's such a great film I'd go and see one if it was made.