If there is enough of a story to follow, I don't see a valid reason why it has to be "short". The way I have planned out the story, there is more than enough substance for a 2 hour feature.
Like I said; This is just a small event that brings the two species together. A story that is set in the AVP universe. But I feel there is enough to make it feel unique and not just a typical "vs" movie with no story behind it.
Yes the concept of a movie with only the Aliens and the Predators has been mentioned many times in the past. But has anyone really come up with a storyline to make the concept work? Well I strongly feel that I have.