Zemekis said:
The problem I see with the Chamax or Alien's for that matter: They are not smart. They don't do interstellar travel. They can be outwitted by a human anytime. So the threat is pretty much localized. Save for some "oh, they caught us by surprise, yet again" adventures I cannot picture them being a galactic menace.
Making them smarter will instantly raise the question: Why didn't they breed/clone their food/human incubators in the first place?)
Smart doesn't always mean human motives.
Still, what's more disturbing is to use a non-intelligent threat like the Chamax, yet have them sometimes "act smart." FREX, what if the Chamax horde were being directed/manipulated by an intelligent force? A couple of ideas - one of the megacorporations is developing an "easy" way to defeat them, and so is spreading them at first to test their new product, then later to eliminate enemies, clear planets they want for other reasons, etc?
Or the Chamax are client species of another race who uses them as the shock troop forefront of an invasion.
Once the players realize there is an intelligence behind the mindless horde, they face an even more troubling problem - how do you glean the intelligence data of who's directing them from critters that are basically mindless?
EDIT: I just went back and re-read your original post. What it seems you are looking for is something that will by necessity create conflict with humans, but isn't driven by the need to just conquor.
What you need is an alien species whos needs conflict with the needs of humans in a such a way that conflict is inevitable, unless a mutal solution can be agreed upon. A few ideas:
- a race that needs a certain spectrum of starlight, and is expanding into systems that have stars with that spectrum. This one can be fun as there might not seem to be any sort of distinct logic or pattern to their incursions at first.
- an interstellar race that is hurt by the mechanics of jump drives. The race could be migratory, or just fed up with having fewer and fewer places to hang out.
- a race that lives in Gas Giants that is expanding their empire and their colonies are threatened by normal human fuel skimming activities.
- a migratory race without FTL that is millions or billions of years old that picks up every hundred thousand years or so, and moves onto the next set of planets on their route around the galaxy - ones that happen to be inhabited by humans now.
- glalactic farmers. A race that has an empire a long way away that regularly terraforms planets to be huge farms. They need planets of a specific type, with specific spectrums, and once they find those, they raze all life, and begin to grow their Kudzu like crops. All their farming is pretty much automated, and they have a disdain for "lesser" life forms.