I found out recently that when they were filming ESB there was a false page in the script that said Obi-Wan killed Luke's father instead of the famous I am your father revelation. Someone on youtube (not me) even made a mockup of what the revelation would have looked like.

Personally, I think that the I am your Father revelation is much more shocking but I can't help but imagine an alternate trilogy where Luke's father and Vader were different people. ROTJ could play out pretty much the same except with Vader realizing that he owes it to his old friend, Luke's father, to make sure his son doesn't fall to the dark side of the force. As he watches Luke, he could remember his friend's death. For some reason, it seems to make more sense to me that two Jedi turned to the dark side and not just one.

Anyway, which do you think is a more shocking twist? If they had used the Obi-Wan, what different directions would the third movie have been lead in? I'm just curious about everyone's thoughts on the matter.