The Star Wars story was finished back in 1983 when Return of the Jedi came out. There's no more story left to tell, at least not in movie form. And the simple fact is that there were many more problems with the prequel trilogy than Lucas being boxed in, timeline-wise. The movies were horribly made. The way they were shot was infantile and incredibly boring, the special effects were cold and unpleasant, and the writing was absolutely inexcusable for something that cost over a hundred million dollars to make. Come on, people! He already broke our hearts with the Phantom Menace! Don't go back to him so he can do it again!
The thing about the Star Wars universe is that it is really not that deep. It's big and epic--galaxy spanning civilizations, crazy aliens, a mysterious energy behind everything, etc--but an awful lot of things in it are more interesting when viewed from afar. Take the Jedi, the thing everyone recognizes about Star Wars. One of the reasons that the prequel trilogy was so awful was because it took all the mystery out of being a Jedi and learning about the Force. In the original trilogy Luke never fully understood the Force. He didn't really become a Jedi until he defeated Darth Vader at the end of the movie. Those movies were not about Luke BEING a Jedi. They were about Luke BECOMING a Jedi. Becoming a Jedi is an epic journey filled with wonder, excitement, and adventure. Being a Jedi is tedious and droning. The Force actually becomes routine and predictable. Force Lightning was the climax of Return of the Jedi. But in the prequels its apparently a routine power that all Dark Jedi above a certain level learn, like the Fireball spell in Dungeons and Dragons.
Take the Galactic Senate: it's much more interesting to hear about them doing things, to hear about the latest laws they've passed, or to hear that the Emperor has eliminated them. But when you actually see them, and witness all the boring crap that they go through, it loses its epicness and becomes boring. The phrase 'Galactic Senate' sounds really cool. It should never have been put in a position where it becomes boring.
Take Darth Vader: he was much more interesting when we didn't know much about him. What was intriguing about him was that he was so mysterious. He wore a creepy mask, had all kinds of dangerous unknown powers and knowledge, and had this ominous agenda that no normal person could ever understand. Ben Kenobi's summary of Darth Vader in A New Hope (he was a young Jedi pupil who succumbed to the Dark Side and killed off the other Jedi, including Luke's father), even though it only took a minute to relate, was way cooler than the entire prequel trilogy combined, because it was mysterious. There are some things that the audience should always have to wonder about--what happened to Darth Vader that made him turn to the Dark Side? The question is much more interesting than the answer.
Lucas can do what he wants. But I won't be checking out any new Star Wars movies until I can get them for free.