I held to the thought that Vader was lying to Luke to mess with his head. The Emperor and Vader talked of turning "the son of Skywalker". Anyway, Obi-wan wouldn't lie.
And going off of early production photos of Luke in the black outfit (especially ones showing Luke standing with Vader) had us guessing that a clone of a dark side Luke was 'the other'. That, or a brand new character not yet introduced would answer the big question, --- but not Leia. Vader would of coarse had sensed this when he had her on the Death Star. We kids knew that was just silly.
Also, a side note, I did feel strongly that when Lando looks over the buttons on Hans Carbon freeze unit, we were sure that he set a timer to free Han so he could escape Boba Fett.
What innocent times, and to think Lucas had no more clue of how to tie up the Trilogy then we did.