I agree, but then it would have to be something big to make him come to his senses, he can't just snap out of it. Maybe after Luke has turned to the Dark side and is seated with the Emperor, Vader realises that he's been a pawn all these years and it's him that turns Luke back, so in the end it was Vader who saved the galaxy. But that means that everything Obi Wan and Yoda taught Luke was a waste of time...So if Luke turned, what would happen to make it only a short term turn? Maybe another way to handle it would be Luke almost turns to the dark side earlier in the story. Maybe he could use excessive force during the rescue of Han (I always felt like ROTJ was like 2 clear, unrelated halves of a movie - The Jabba bit and the Endor bit. This would provide a link to the two pieces). He could lose his temper at times instead of being the chilled out goodu goody farmboy for the whole trilogy, so it looks like he might possibly turn out like Vader, but he overcomes that pitfall and emerges even stronger, ready to take confront the Emperor and redeem Dad Vader, and then the end could go just like it does in the movie because Luke has already faced the temptation of the dark side and overcome it.