Apparently, George Lucas originally intended for Luke Skywalker to become Darth Vader at the end of Return of the Jedi. His last words were going to be "Now I am Vader. Now I will go and kill the [Rebel] fleet and I will rule the universe."
Oh man, that would have been so cool.
It's one of ten unknown facts about Star Wars. This year marks the 30th anniversary of Star Wars: Return of the Jedi. To celebrate, Star Wars historian J.W. Rinzler created another coffee-table book, filled with secrets that were never revealed about the making of the film.