Characters: I would like to see an elite military member from a tier one unit engaging in clandestine work. DEVGRU, Seals, Green Berets, Detachment Delta, CIA SAD, Etc. These guys should be the best of the best, and maybe they should get some real veterans in there. The goal should be to have them look as much like real tier one operators as possible. Hand to hand combat would be cool, but it has to be realistic. I liked the ending with Arnold in the first Predator because it didnt make Arnold look too macho manhandling a Predator, and also Arnold had to resort to his basic survival skills once he lost his weapons.
Setting: Some specific places I think would be cool: Middle East (Iraq, Afghanistan, Yemen, Etc.), Africa (Somalia, Darfur, Nigeria, Congo countries in the diamond areas like Sierra Leone), North Korea, Balkans. Instead of one Predator and one location which is what all Predator and AvP films have been, it would be cool to break it up into where there is a primary location and maybe some other secondary locations around the world.
Plot: Would be nice to see Arnold and Danny Glover return for a cameo or supporting role... maybe they have become friends and are working together to learn more about what the government is doing to find the predators and get their hands on predator technology. They could be doing private investigations into predator sightings so they can track one down themselves, and learn more about the predator species.
Maybe the main characters should be the tier one unit sent by the cia, military, and military contractors to hunt down and capture or kill a Predator to study its technology and physiology. Or to imprison and attempt to communicate with it.