Yes, I'm aware this was the introduction of the Xenomorph in general, and nothing was known about it before this movie. Even if there would have been further movies after it, there's no way those movies could have been anything like the ones we know.
Ripley would be dead, and the Aliens would be a lot different.
Probably the sequel would have been something like.... the Alien lands on earth in the shuttle and begins colonizing/assimilating earth by morphing people into eggs, imitating their voices, or even has the ability to morph into people and pose as them.
Or, another sequel could be a team being sent to LV-426 to find out what happened to the Nostromo, and the team are all infected with face huggers. The aliens are then born and they begin taking over the planet, building technology, or even taking off in the Space Jockey's ship.
Doesn't sound too good either way.
How 'bout a sequel where the Alien lands on earth and morphs into a person. The Alien then adapts to life on earth by working in some office building and getting adjusted to human customs. Or the Alien lands on earth and feels bad for Ripley's daughter Amanda, so it poses as Ripley, pretending to be Amanda's mother?
Could make for some good sitcom spinoffs...