I think it would be really lame if Luke went to the dark side. I mean seriously? Wouldn't it kind of render all of his sacrifices, including risking his life to save his father, kind of pointless?
If Luke could resist the temptation to turn when his own father tried to sway him, and even DEFIED the Emperor (who as far as we know is the penultimate form of all evil in the SW universe) by NOT turning, but believing that his father loved him enough that Anakin could be saved, then who or what could possibly make Luke betray that belief? If Luke was going to fall, it would have been at the climax of Return of the Jedi. He would have killed Vader, killed the Emperor, and declared himself ruler of the universe. But he didn't.
It would completely diminish his entire character to do that now in the new films. That's the reason why I never got into the Dark Empire storyline, and felt it was weak writing. Besides, that story was already told in the Prequels with Anakin's fall. Do we really need to retread that plotline?
