Well, the Alien and Predator franchises have already been kind of tied together with the AvP films whether we like it or not. So that's already been kind of done despite the fact that a majority of fans don't like the AvP films.
Also, Prometheus and PREDATORS kind of have a dubious route in continuity so tying all of the films together more than likely can't work.
-Rakai'Thwei