While many fans believe that fifteen seasons was too many for Supernatural, the show did find - or at least did try to find - a lot of different ways to spice things up and breathe fresh air into the lives of Sam and Dean to keep the show exciting. Whether through plot twists or changes to mytharcs or even wild concepts for episodes, fans were often kept on their toes.
Of course, not every single surprise and shock addition to the show improved it. Whether good or bad, there is no doubt that so many unexpected things happened on Supernatural that not even the most passionate members of the fandom could have seen them coming.
Certain Deaths
The show is chock full of deaths, and while it is a bit of a meme that characters often die only to return later on, some Supernatural deaths were permanent. Not only this, but some characters' demises were utterly shocking even if a return in some form was on the cards.
The deaths of Bobby, Kevin, and Charlie shocked so many fans and remain heartbreaking despite all characters returning. The death of Crowley being so final and diluted still irritates and surprises fans. There are, of course, many others, but perhaps the most unexpected was Dean's final death, not because every fan expected him to live, more so because nobody expected him to go out via a nail/rod stuck in a wooden pillar.
The Way In Which Heaven & Hell Were Run
The concept of hell had been a part of Supernatural since season 1, but it got depicted as a torturous, chaotic pit in those early seasons. However, later on in the series, particularly when Crowley took charge, it was shown to be run like a business or political office.
There was obviously still torture, and the souls were kept in cells, but having demon patrols and seeing such bureaucracy definitely surprised many. The same can be said about heaven. When it was introduced in the show, many were intrigued, but as it turns out, bureaucracy ruled there too, with angels having no genuine care for humans and running it more like a business.
Dean Killing Both Hitler & Death
Dean Winchester got some high-profile kills throughout Supernatural, from Azazel to Zachariah to Abbadon to Ruby to Dick Roman - the list goes on. However, undoubtedly his most unexpected kills were both Death and Adolf Hitler.
Death as a character was not a surprise inclusion to the show. His getting murdered, though, definitely took many fans aback and saddened them; they wanted to see more of Death and Dean as they became a great Supernatural duo. Hitler, on the other hand - and the presence of Nazi necromancers in general - was a bold concept for the show and one that not many fans saw coming.
A Full Winchester Reunion
Another of the shocking deaths in Supernatural was that of John Winchester in the first episode of season 2. Fans never expected to see Jeffrey Dean Morgan reprise his role as John, much less so during a time when Mary was also alive.
Alas, the stars aligned in season 14, and the Winchester family got to reunite. It was unexpected when Mary returned, but when John came back for one episode only - Supernatural's landmark 300th episode "Lebanon" - fans saw something they never thought they'd see in the show, a happy and complete Winchester family.
"The French Mistake"
There are few, if any, shows that do meta like Supernatural, not quantitatively nor qualitatively. "The French Mistake" is one of the show's best efforts in that regard and an episode nobody saw coming.
It sees the brothers leave their own reality and arrive in one where Supernatural is a TV show that stars Jensen Ackles, Jared Padalecki, and Misha Collins, where Jared Padalecki is married to the actress who plays Ruby II, Genevieve Padalecki (née Cortese) - a.k.a the real world. It is a concept that only it could pull off, becoming one of Supernatural's best and most rewatchable episodes.
The Wizard Of Oz Being A Reality
The Wizard of Oz is a story that has and will continue to stand the test of time. It's also a story that nobody expected to pop up in Supernatural, save for a moment of Sam's nerdiness.
Of course, when the world of Oz and the characters who go along with it, such as the Wizard, Dorothy, and the Wicked Witch, are introduced in season 9, it is a different take on the mythology. As weird and wonderful as Supernatural could be, it was still a surprise for fans to see this story told.
The Men Of Letters
The introduction of the Men of Letters changed Supernatural a considerable amount. It not only added a lot to the Winchester family history, but it gave the boys a home, aided in the expansion of myth-arcs, and helped usher in bold new concepts to the show.
On top of all that, nobody saw it coming. As far as fans were aware, hunting was simply hunting. Individuals or groups spread across America or the world and killed monsters. Nobody thought there was a secret society of academics studying and collecting information and lore on all of these unnatural beings. It was a fantastic addition to the show nobody saw coming.
Jack
When a show that primarily focuses on two characters, and then later two, runs for thirteen years with its time clearly winding down, nobody really expects a fourth to get added into the mix. But with Jack Kline, Supernatural did just that.
When Crowley died, Jack took up the mantle of the fourth central character, a decision that fans still debate today, arguing over whether or not he was a good or needed addition to the series. A new recurring character would not have been a shock, but one that was pushed in the spotlight as a central character, arguably more than Bobby or Crowley ever were, certainly took fans by surprise. There are lots of fans who do love Jack and see him as one of Supernatural's best characters, so perhaps it was a good move.
A Scooby-Doo Crossover
Scooby-Doo and the gang are actually referenced a few times throughout Supernatural. Dean had quite an affinity for them growing up. But fans never dreamed that the Winchester brothers would ever find themselves trapped in the Scooby-verse.
"Scooby-natural" proved to be one of the most unruly yet successful concepts that the Supernatural writers ever came up with. The episode was hilarious, blending both worlds brilliantly. Nevertheless, watching Sam and Dean run alongside the Scooby-gang when a decade earlier they were startled by a demon hunt is quite astonishing.
Chuck, Author Of Supernatural & God Himself
Even when the monster-of-the-week style of the show was ongoing and angels had just been introduced, Supernatural was not afraid of meta nor out-there concepts. Enter Carver Edlund, otherwise known as Chuck, the author of the Supernatural novels that detail Sam and Dean's lives and hunts.
This on its own was a tough pill for fans to swallow. But seven years later, the idea that the author of these books would be revealed to be God seems an even more unlikely direction. However, that is exactly what happened, and Chuck became the final big bad of the show's run. Some fans theorized that Chuck was God back in season 5, but back then it just felt like a crazy theory, so viewers were even more shocked when it turned out to be correct.