This is going off topic but here is my idea for how a team up might occur...
In a callback scene to the first film, a predator lifts a colonial marine by the throat, pins him to a wall, and looks him in the eyes. He draws his blades and it appears the marine is finished, but then... his motion tracker starts beeping. The predator glances at the tracker and knows what it means: xenos are about to come calling. After a moment of consideration, the predator releases the marine, not giving him another look as he gears up. The marine picks up his pulse rifle, reloads, and takes a position next to his new ally. It's game time.