I predict that the Rowena will 'help' Sam remove the MoC from Dean, only for it to transfer to Sam's arm instead. That seems to fit with what the showrunners tend to like.

Myself, I would have gone for Sam discovering and using the residual Lucifer grace left inside him to alter the MoC, leaving Dean paranoid about exactly what that implies. But that's probably to subtle for the season finale.

Eponymous