I think the shot is great, with the predator. I think it would be best for all the franchises if in the actual "series" movies, they don't do crossovers. In AVP3, then we have a chance to include all three, but the story lines of each independent series shouldn't have to include anything from the cross-over movies.
If they are going to make something very good with the predators in Prometheus, though, as I think they may have already (and dropped clues to it), then I'd say get ready for them to fill a rather big role. They're real game changers for the engineers.