Okay, so someone above had the silly notion that he used his ideas planned for the sequesl in ROTJ. Quite wrong. It is pretty obvious from reading all the drafts that ROTJ just expanded on the very shortened epic battle Lucas had planned originally. Basically Luke’s training and Bespin, and then all the stuff on Endor would flesh out what was truncated into the Falcon training, the Death Star scenes, and the final battle (which would have been a surface and space battle). Lucas has said as much many times.
The only hint I’ve ever heard was from Mark Hamill who asked if he would want to come back as Luke in 30 years and play the Ben character in a new trilogy. Outside of that I have never heard anything credible.
I think had Lucas done the ST that it would turn out different than the PT. A lot of the PT’s problems are with the basic story. Political stories like that just aren’t as popular. I think his vision of the PT was always going to be inferior to the original because it is less epic and more involved. The PT would have been better if he had brought in a good dialog writer to co-write the scripts.
The ST might have a more epic and interesting story, but I think it would have suffered from the same writing mistakes that plague the PT. It would have had Luke, Han, Leia, Chewy, Threepio and R2 and it would be hard to screw them up so I think the GL ST would have turned out better than the PT, but not as good at the OT.
I think the best scenario would have been to take his story ideas for the ST and turn them over to competent writers to execute. Keep his arcs, fix his characters and the details and continuity of his plots and I think it would have been fantastic. I think some of the biggest mistakes in TFA are that they played it too safe. They hired a director who would hold true to the fan image of the OT and they hired the TESB and ROTJ screenwriter (who did not come up with the stories - Kasdan cold have done miracles with the PT). Between them they came up with an acceptable plot, but one that relies on some of the tropes of the first film (though I think they managed to tell a very different story with them). I think GL’s story ideas would have given us the more typical 3 separate stories that form a trilogy instead of the second almost being a continuation of the first.