I was watching the Alien Saga special on AMc a few days ago, and it came to me that the Aliens franchise was in a similar state as The Godfather is in now. Two megablockbuster first movies (Alien, Aliens) a lackluster threequel (Alien 3) and a much acclaimed and original fourth (Alien: Ressurection)
The people behind Aliens decided to reclaim the films status by making a fourth, because the third was not well recieved. They got a whole new writer for the treatment and director than the other three and bam you have yourself a hit movie. They also enlisted the services of a bunch of acclaimed character actors. Even though Godfather and Alien are two very different movies, I think if they used this method at making a fourth it could work, just a thought.