There are a lot of interviews in the past where George said he had enough ideas to make both a trilogy before and a trilogy after IV, V, and VI, but as far as those ideas actually being scripts that he just shelved in favor of those three original movies, that's just not true as far as I can see. I'm not one to usually promote the DVD set, but you really should take a gander at the Empire of Dreams documentary. It does a very good job of explaining the origins of the story and the different changes it went through before it became film.

For example, if George's original story was I-VI or even I-IX, why would only IV-VI be so close to trying to mirror the ancient archetypes of mythology, which is one of the main things that motivated him to come up with the story in the first place?