I think this is kind of what Fox and Ridley was saying about the potential of the franchise and that it does not have to be about Xenos....
so yes we could see a spin off about the Marines and Weyland-Yutani that could be done and not have nothing to do with the Xeno so could be set before Aliens or even After, if we assume that there is nothing left on LV 426 as far as Aliens canon goes.
Likewise Prometheus could open up another franchise, you see are Humans on Earth the only seeded Humanoid life that the Engineers had created? Is Paradise and LV 223 the only places where you find Engineers in our Galaxy. They could explore another Humanoid race or even Humans in another part of the Galaxy that has no contact with Earth, such as Battle Star Galactica etc.
These Engineers could be alive and well and interacting with other creations in a far far place in the Galaxy that is simply beyond the means for Weyland-Yutani or any other Earth Company to reach by Star Ships even in the timeline of Alien Resurection.
They could alternatively do a movie that is based on Engineers and their creations or even Earth but one set thousands of years ago.
But back to the topic of doing a Alien Timeline i.e 2100-2200 movie just about the Colonial Marines, why not? we cant rule it out.
And i would pretty much Garuntee we would see a Comic Book, Novel or Game based on them that has no connections with the Xeno, Engineers or Predators.