I would like to get back to the three questions that Gebohq raised in his (or her) first post. Although I do agree with most of the stuff that already has been said in this thread, I have a few things to add. Considering the fact that the Swiss artist HR Giger designed the Xenomorph, the Spacejockey and the Derelict in the first Alien movie, we know that there are biomechanical beings in this setting. In Prometheus, it was revealed that the Engineers once seeded life on Earth. Just by considering that this stuff is a part of this universe, it's possible to answer the three questions in a different way. Here is my take on life in the Alien universe:
1) Is there any non-sentient life outside of Earth-based life in the Alien universe?
I would say that the Engineers and/or the Spacejockeys probably have been capable of seeding life for a very long time. It might be a part of a very large, interstellar or even intergalactic process that has been going on for thousands, or maybe millions of years. What we call terraforming might just be a very primitive form of the processes that others are capable of. Most certainly, they are able to bioengineer life in forms that are unknown to us. The creatures and plants we see on Earth are just examples of they can do. Considering that life on Earth was once seeded, similar forms of life might be found on other planets, because the creators were probably the same or at least similar. That would explain why the planet in Alien Covenant looks very much like Earth. I know it's obvious, but it hints how other planets might look: both very familiar and and completely bizarre (after a closer look) .
2) Is there any sentient life outside of Earth-based life and the xenomorphs (and their variants) in the Alien universe?
On Earth, plant life is a part of a large ecosystem that also include many kinds of animals (no surprise...). Knowing that life here was once seeded, the logic of life on other seeded planets might be the same or similar, even though life might have been developed in other ways. It could be a question of what a planet is seeded for. The design logic could differ a lot between planets, or maybe the design protocol didn't work according to the initial plan. The Engineers (or the Spacejockeys) could have made visits and tried to correct things, as they probably did on Earth (at least someone visited Earth, cave paintings hints that). Life is a matter of design – successful or not. The results of this design is probably be what the visitors from Earth will encounter when they travel to new worlds.
3) Is there any sapient life outside of humans (and space jokeys/engineers) in the Alien universe?
Maybe there is, but I would actually like to rephrase the question into two new questions. First: What does it mean that the Engineers are more or less the same as humans on Earth? And: What does it mean that the Xenomorphs, the Spacejockeys and the Engineers all are quite similar beings (arms, hands and heads)? Again, I would say it´s a question of bioengineering. They all come from the same (?) design or even the same biomechanical ecosystem. Of course, it's also possible that they are the result of some kind ancient encounter between totally unrelated beings from different parts of the universe. As we know, there is something Lovecraftian in the entire setting (and in in the original Gigeresque vision): humans on Earth might be very insignificant beings in a much larger intergalactic context where there are (or once were) other, more divine beings (or they just appear so to us). I also prefer to see the difference between various creatures as something fluid, a matter of design. That means that there might very well be other kinds of humans who differ a lot from us. They might, for example, be more like insects or partly biomechanical (like the Xenomorphs or the Engineers, who seem to be integrated to their suits). Those alien humans might be the original, if there even is an original. Again: life in the Alien universe is, I think, a matter of design and bioengineering. Everything can be edited and manipulated in forms that are hard to imagine. You could also question what sapient life is and what is not (it might be irrelevant in a larger intergalactic context). Or to put it differently: are humans even an intelligent species? The creators behind it all could be completely different themselves, and maybe they got extinct millions or even billions of years ago.
As others said in previous posts, sapient life in the Alien universe is probably not that common. I would assume that what the PCs might encounter is more likely remains, bizarre archeological fossilized stuff, similar to (or very different from) the Derelict in the first movie. To just invent more alien species is, as I see it, simply not that creative. It has already been done in too many other RPG settings (adding another half orc and so on). To me, it's a lot more interesting to expand on the ideas and visions that are already a part of the Alien franchise. To put it short: imagine that there is an interstellar or intergalactic biomechanical ecosystem (I think it has been hinted through the very existence of the Xenomorphs) and develop new stuff (creatures and biomechanical technology) from that logic. That is something else than creating a staple of new B movie monsters (which I think is tempting but not very interesting in the long run).
Finally, I´m not saying that the total story of intergalactic life should ever be written within this franchise. That would just kill the the mystery. But I do think it's thought provoking to sometimes get glimpses of the bizarre reality that might be out there. It should, I think, feel truly different, alien, horrifying, ancient and realistic. That is what makes this entire franchise interesting in the first place.
Most of the stuff I wrote here is actually just a part of the much more fleshed out vision that I presented in this earlier post about the HR Giger legacy in the Alien franchise:
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