And of course the whole PIE space aliens angle was not further explored in Covenant.
The whole Alien/Aliens franchise was never supposed to be true intellectual sci-fi. From the beginning it was about and always was meant to be about scary ugly aliens eating/attacking humans in disgusting or horrendous ways. That's it.
The only thing I liked the most about about Prometheus (besides the Engineers) was the star map. You actually do find patterns like that in the world but as to why they are there... the conclusions you can draw about them are various.