My idea, if it was to be set in the future is that it could be set anytime between Alien 3 and Alien Resurrection.
Weyland Yutani has grown into the largest corporation in the planet (thanks to the technology developed from the Predator weapon in AVPR) and Earth is now their main HQ.
A Predator ship containing face huggers crash lands (couldn't think of anything original here sorry) on earth and Weyland Yutani captures them and secretly begins breeding Aliens in the hope of using them as the ultimate weapon.
However the Predator homeworld soon learns of these goings on and sends a group of Predators in to eradicate the threat.
However it does not go to plan and the Aliens escape, and begin an onslaught against the humans inhabiting the planet.
Weyland Yutani orders all of their colonial marines to battle and the film should finish with a 3 way battle between Aliens,Predators & Colonial Marines.
Then the film should end with the destruction of the earth and Weyland Yutani. (i.e. merging with the story of Alien Resurrection)
Anybody like my idea?