If they do turn Will into some kind of mythical creature in #3, I doubt it would be to enable a Jack/Elizabeth finale. I don't think anyone (involved in the movies) sees Jack settling down with anybody ('cept maybe the Pearl). That might be why Johnny keeps saying that he'd be willing to continue with a #4, while the writers feel they're closing up the "Pirates" universe with #3; maybe the only characters "finalized" are Will and Elizabeth (though I still can't imagine what they'd have in mind - specifically - for Elizabeth, if they do turn Will into a "Davey Jones" type character.