What's in store for the "I am you father" revelation? Perhaps Darth Vader could have an identity crisis, and reveal himself as "mother", "sister", "the Emperor", or "Obi-Wan" at various points in the trilogy.

Or he could confess that he killed Obi-Wan, with Luke being under the impression that his father had killed Obi-Wan.

EDIT: I just realized that this doesn't work nearly as well if Obi-Wan doesn't actually die onscreen at the hand of Darth Vader. I don't suppose there's a way they could leave him behind, then go back to get him before the Battle of Yavin, only to witness his death?

And sorry for not keeping my ideas in the appropriate threads. :P