Ok, can ANYONE come up with an "Alien" prequel plot that sounds remotely interesting? I've been giving it some serious thought, but I've got nothing... I suppose The Company knew that the warning signal was there, maybe an earlier vessel encountered it and no-one survived, so they sent The Nostromo, but it seems totally unnecessary.
This idea, about an earlier crew discovering the alien xenomorph, is probably the basis of the planned prequel. But I sincerely doubt the "no survivors" angle. Someone survived long enough to bring back the data that convinced the company that sending another ship to investigate (the Nostromo) was in their best interest. And maybe not just data, but perhaps even a specimen.
I figure that this earlier mission probably wouldn't get a whole lot of screen time, as its primary purpose would be delivering the suspended xenomorph specimen to a Company space station in Earth orbit. The specimen is revived, but before The Company learns much of anything of value about it, the xenomorph escapes from quarantine. Dozens die, and The Company finally opts to "sacrifice" the station, and all aboard, to prevent the xenomorph from reaching Earth. (Not to save Earth, mind you, but to prevent a possible asset to fall into the hands of a competitor.)They figure that they'll simply send another ship to acquire a new specimen.
The film would then end with a secret meeting where The Company decides to replace the Nostromo's science officer with an android pre-programmed to obtain a new specimen on the planet where the xenomorph was first encountered.
Speculating further: Maybe that first group to stumble upon the "Space Jockey" ship didn't just bring back a living specimen of the xenomorph, but some space-jockey technology as well. Like maybe that strange vessel's flight recorder, which contains the location of the xenomorph's probable planet of origin. This would set the stage for a later sequel, which might even take place at the same time as the original film, (a plan "B") where another unsuspecting ship is sent to obtain specimens from the alien homeworld itself.