Here's my ideas.

For Alien 5 Ridly Scott and James Cameron should team up for that one.

For Predator 3, Strause Brothers and Steven Hopkins.

Morgan Freeman and Samuel L. Jackson are good, but maybe not the main-main character of a Predator movie. Maybe someone like Jet Li's adoptive father in Unleashed (Morgan Freeman) or Mace Windu in Starwars (Jackson). Important but not the main character.

someone said in an earlier post saying they wont make an alien 5 as its a quadrilogy now, well i bought scary movie trilogy then they brought out a 4th lol, there's no stopping a director making an alien film i reckon, though it has to be a director who KNOWS what aliens are like and read the comics, if they make one i don't know whever or not it will be on earth because avpr is set in our time and then that gives you a realization that the alien quadrilogy is all set in the future lol and avpr is set in the present :S anyway Sigourney Weaver better be in it if it happens and here's a link of a story someone made up which sounds ok http://www.countingdown.com/movies/546775/news?item_id=3086970
As for predator 3 it should have a new actor that we don't know as it will give predator a new feel to the movie, new start, maybe something could be set after both the films with a charactor associated with the organisation that was in the 2nd pred film, cant remember the name of the corporation though lol i'll stop talking...