I think there's much more that can be done with the Aliens than the Predators.
Consider that Predators are based upon earth's own warrior and hunter cultures. They kind of make logical sense to us earthlings because of that, and therefore there's not a lot of exciting ground to cover.
With the Alien creature, it's not so much about the creature itself but interpreting it. Remember in the ALIEN series, the creature served as a dramatic tool to cause social dissonance amongst the characters, driving some kind of drama rather than just being some kind of beast to kill.
An ALIEN prequel can do a lot. I love the idea of a corporate spy thriller using Weyland-Yutani in particular. It could explain a bit about The Pilot, too, especially if Weyland-Yutani collected data about the race and, on specific purpose, juiced up the warning signal we see in the original ALIEN. That would imply that they were studying the Space Jockeys as a way to come closer to the mystery of this amazing bio-weapon. The movie could end with a character messing with the Jockey ship, boosting the signal.
