Predator 3 plot ideas
To make it memorable I think Robert has to go backwards. Maybe focus on a Mayan civilization. Although he should keep it in a low scale, in other words don't allow the whole civilization to be aware of the Predators existence, or at least not until the end of the film. If you take it back to the origins of the Predators it would be far more memorable than some big budget, high tech saga, involving humans with lasers and spaceships. The Predators always need to have one up on the humans.
So taking it back to their origins will not only be more entertaining, they can also go into to detail of how exactly the Predators found and came to Earth. This would not only tie in with the stories described in the first AVP (predators came down and built the pyramids) but it could also raise a lot of controversy in the area of debating the existence of aliens and the possibility of them at one point in time coming to earth to teach us how to live and build.
Now you may be thinking how is that going to make an entertaining action film, but the best way to get this across would be to have a small group of Mayan warriors who fight off the Predators. The Predators must prove their superior knowledge and technology. After killing off the warriors the Predators make themselves known and then proceed in teaching the humans....end