Supernatural is known for endings its seasons on crazy cliffhangers, and the last season managed to pull off another one. The final episode of Season 13 saw Dean finally give in and allow the archangel Michael to take control of his body as a vessel. As we've seen in the past, acting as a vessel doesn't mean anything good for the person whose body is being taken over, and Dean won't be acting on his own volition to at least start Season 14, and that means Dean will be missing. Supernatural showrunner Andrew Dabb spoke about Dean-less Supernatural, saying this:
[O]ne of the pillars of the show is gone, basically, and that, in some ways, makes things very difficult, because the show is Sam and Dean, and their interplay.
From the very beginning, Supernatural has been the story of the Winchester brothers saving people and hunting things. Without both Sam and Dean, Supernatural surely would have gotten the axe by this point. The brothers Winchester are what make the magic happen and sustained the series for more than a decade; what will Supernatural look like with only one Winchester in a given episode?
Dean delivers a fair amount of Supernatural's humor in the monster-of-the-week stories, and I'm not sure that any of the characters who might step into his shoes while Michael is inhabiting his body can deliver the same kind of laughs. Admittedly, Supernatural has gone without one of the brothers for large chunks of episodes, such as when Sam and Dean weren't getting along and went their separate ways for a bit. Still, an altogether absent Dean for multiple episodes is something new. Hey, at least it proves that Supernatural is still capable of reinventing itself after 13 years (and no spinoffs), and it does mean a meaty storyline for Jensen Ackles after Dean's demon arc ended rather quickly.
Dean's absence will presumably mean more opportunities for characters like Castiel, Jack, and Mary to shine. We do know that the Wayward Sisters will turn up in the new season, having lost their spinoff. Sam can't exactly investigate everything on his own, and he may miss things in his investigations if he doesn't have somebody to bounce ideas off of. Even if none of his friends can do exactly what Dean did and occupy his space as Sam's relatively codependent brother, Sam will undoubtedly have help for however long Dean is stuck as Michael's vessel. Andrew Dabb went on in his chat with TVLine to say this about handling episodes without Dean:
It's challenging to write episodes without Dean Winchester in them, because he is such a fundamental part of the show. But it's also exciting [for] Jensen and us [to have] the opportunity to play around with this new character.
For a guy who has been playing the same character for more than a decade, Jensen Ackles may relish the opportunity to stretch his acting muscles and tackle a new role without actually leaving his show. It should be fun to see the differences between his performances as Dean and as Michael-in-Dean. A big question is how long Dean will remain Michael's vessel. Supernatural surely isn't going to sideline one of the Winchesters as a vessel for however long the series runs, and fans may not be happy if Dean is gone for too many episodes.