What if there is a world where there’s a bad Dean Winchester and all Dean has to do is to get Michael into the other Dean and then kill them both? It wouldn’t be difficult, because our Dean puts up a fight and that’s something Michael doesn’t like, so I guess he could eventually jump into his other “perfect vessel” who’s not only HIS vessel, but is also dark and evil. It would make things much easier, in Michael’s eyes.
I mean, in 14x10 Michael said:
God – Chuck – is a writer, and like all writers, He churns out draft after draft. My world? This world? Nothing but failed drafts. And when He realizes that they’re flawed, He moves on and tries again
We know there are other universes. AU!Michael comes from a world where Dean Winchester doesn’t exist only to posses his “perfect vessel” and destroy Dean’s world. He will then move on to other worlds to try and catch God - or at least get his attention.
We know there are alter egos, like AU!Castiel and Dark!Kaia, who killed her “original” self and still stayed alive, so there’s no tricks like “if you kill your alter ago, you’ll die yourself”. As far as I recall, dark!Kaia doesn’t come from the same world AU!Michael does, so there must be at least one world in which Dark!Dean exists. They never dismissed the idea.
What if the book Billie gave Dean says “Kill yourself”, and of course they won’t reveal the solution because we have to suffer in drama first, but it would eventually turn out it doesn’t have to be your original self - aka loophole.
It would make beautiful sense on the meta level - Dean killing his bad self, everything he despises and doesn’t want to be. It would also make sense why they show Michael in Dean’s vessel even in Dean’s mind, whereas they showed Mark inside Cas’ mind and Tahmoh in Sam’s mind AND why Dean talked to the mirror - one, to make sure it’s just him and not Michael, but what if it’s also some kind of foreshadowing in which Sam and Dean will have to guess which Dean is which?