I'd like to have Ben's ghost at the final duel in ROTJ, just as a vision tho.
So that Vader sees it and remembers of his friendship with Obi Wan... and so that Palpatine sees it the moment before dying, he sees how through the bright side you can achieve some kind of inmortality.
Besides, Vader could be a Jedi again, and whatever, but this doesn't imply he's in good terms with Kenobi after his death...