Gary Kurtz did a really in-depth interview about all this once, explaining why didn't stay on for RotJ, how the prequels and sequels were originally supposed to work, etc. He said that all the material covered in 7-9 was all condensed into RotJ, as Lucas wanted to wrap things up then and there. Originally, Han was going to die, Leia become Queen to the people of Alderaan (?), and Luke's sister was to be hidden in another galaxy. There was no 2nd Death Star, no Ewoks, and the big showdown with the Emperor wasn't until Episode 9. All things considered, I think George did the right thing by ending things the way he did with JEDI. Of course, a few things could have been handled better, such as Wookies instead of Ewoks, the Emperor/Vader/Luke confronation taking place on Coruscant instead of the Death Star, and the 'completion' of Luke's training, but nevertheless it was the right path to take. However, it still left open the possibility of sequels; that is, before the PT came out. Before that, Star Wars was just the chronicles of a galaxy far, far away, but with GL's revisionist history and the introduction of the Prophecy of the Chosen One, it became the story of Anakin Skywalker. Like it or not, that story is completed in RotJ, and any further films are unecessary.

I do have a pretty radical proposal for a new trilogy, though. What would you guys say to three new films that served as the PROLOGUE, INTERLUDE, and EPILOGUE? The Prologue would serve as an origin of the Jedi and Sith, featuring the Sith Wars. The Interlude would cover the time in-between RotS and ANH, with Vader hunting down the Jedi, Obi-Wan's solitude, etc. And finally, the Epilogue would bring us back to our old friends from the OT as they re-build the Republic and the Jedi Order. I know it'll never happen, but it covers everything we want to see without being overkill, and gives us three more movies. Whether they could be GOOD or not, that's up for debate.