There are plans for an Alien 5, but I don't think it will be much good, especially if they choose to drag Ripley through yet another Alien movie...I'm crossing my fingers for a new main character, hopefully with the same calibur as Ripley.
To my knowledge, there are no plans for another Predator movie. I guess they knew when to stop that franchise *shrug*
And thus I clothe my naked villainy
With old odd ends stolen forth from holy writ
And seem a saint when most I play the devil.