there WILL be more star wars movies....from a certain point of view
Aside from the obscene amounts of money involved, the way lucas left so many mysteries in EPIII leads me to beleive two possibilies.
the main mysteries i speak of revolve around darth plageus, sifo dyas, qui-gon jinn, and anakins birth. there is no good reason, from a movie-making point of view to RAISE NEW QUESTIONS in episode III without answering them, let alone answer the question around who sifo dyas is in EPII. sure, it could be explained in a book, but great movies dont leave the books for explainations.
Here are the 3 possibilities:
1. Lucas plans on atleast 3 more movies. they would be set before episode I.
2. Lucas plans on creating "ultimate editions" of all 6 movies and re-releasing them in theatres at some point in the future. He would do small things to make the movies more interconnected, but mainly, he would add scenes in episodes II, and IV-VI which answer the questions around sifo-dyas, qui-gon jinn, darth plageus, and anakins birth.
in my opinion, #3 is the only real possiblity. from a continuity standpoint EPII is awkward b/c you saw sidious all throughout EPI and III, but he's absent until the end of EPII. it doesnt make sense. then, if the saga is really about ANAKIN SKYWALKER, not luke, we need ALOT more scenes of vader and the emperor in episodes IV-VI.
this would be really easy. you can stick anyone in a vader suit, and ian mcdirmid has aged and can play an EPIV-VI emperor easily. we already got a taste of this in the ESB DVD with the additional vader/emperor conversation.
but whatever lucas does, star wars is not over, there are new MOVIE experiences to come.