The first would be considerably more difficult than the latter, especially if you want to associate with Alien.
Which is a universe that is more Dark Souls, full of ruins, strange structures and inhuman remains rather than sentient beings.
You'd have to really, really go abstract if you want their inclusion to be worthwhile so I'm not just asking-
"Why aren't these just humans with cybernetics or humans that generationally were changed by a planet's gravity?"
With that in mind, I'd much prefer to see the Predator hunting monsters both mundane like a giant insect and extreme like a Dune worm or
something akin to the world serpent from ancient mythology.
& Again, I feel the world of the human beings in Predator needs expansion
in that way I can look at an environment from an Alien film and in many ways it stands on it's own two feet even without the creature itself.