I'm one of those rare Star Wars purists who consider ROTJ to be my favorite. Nonetheless, there are a few things I'd change. The biggest change is that Luke and Leia wouldn't be siblings (Luke would be over his crush and Han would win her heart). I don't know who the other Yoda spoke of would be. It would be a challenge to introduce such an important figure without disrupting the story. Perhaps that is a character who should die (for an important reason). The problem of having it be Luke's long-lost twin born of Vader is that Obi Wan seemed oblivious to her importance/existence in ESB. Easiest place to pick up a new character would be in the beginning on Tatooine. Perhaps the mysterious new figure would accompany Luke to Jabba's palace. If I were writing ROTJ, I wouldn't want to do the Death Star again. But how else can we portray the ultimate battle after facing a machine that can destroy planets. There is a pleasant simplicity to doing the Death Star...but bigger! I'm just not sure how to top that. And the climax should top the ANH one. Lastly, I would make the Ewok victory more believable. I'm an unabashed Ewok lover, so I wouldn't ditch them. As I said, I really like ROTJ, so I wouldn't change too many things.