There could be a fanfiction where on Halloween, a portal opens up to a dimension filled with all the monsters they've ever hunted. Sam and Dean are pulled in and have to fight their way through hordes of familiar foes. Along the way, they meet a mysterious figure who claims to know a way out, but they're not sure if they can trust this person. They have to decide whether to take the risk or keep fighting on their own.
A plotline could be that on Halloween, Sam and Dean discover a town where every year, people disappear mysteriously. They suspect it has something to do with an ancient Halloween ritual gone wrong. As they dig deeper, they find that a group of cultists are trying to summon a powerful entity. Sam uses his research skills to find a way to stop the summoning, while Dean prepares for a fight.
One interesting plot is that Sam and Dean are cursed on Halloween. Dean keeps seeing visions of their past hunts in a distorted, nightmarish way, and Sam loses his ability to read any lore. They must find a way to break the curse before it drives them insane. This forces them to rely on their brotherly bond more than ever as they search for a cure.
One great fanfiction could be where Dean and Sam have to face a unique Halloween - themed monster. Maybe a witch who uses Halloween traditions to cast powerful spells. They would have to use their wits and hunting skills to stop her before she causes chaos in a small town.
Another interesting plot could be Sam forming a connection with other werewolves. Maybe he discovers a pack that has different values than the ones he's used to fighting, and he has to decide whether to side with them or stay with his hunter ways. This creates a moral dilemma for him. There could also be a plot where Sam's werewolf nature attracts a powerful and dangerous enemy that specifically targets him because of his unique condition. It forces him and Dean to find new ways to protect themselves and defeat the enemy.
Well, in some fanfictions, they go back in time and accidentally interfere with the lives of their younger selves. This creates all sorts of chaos as they have to deal with the younger Sam and Dean's reactions to seeing their future selves. Or perhaps they go back to a crucial moment in the history of the hunter community and end up changing the rules and relationships within it. It could also be that they go back in time to a specific era like the Wild West and have to deal with supernatural threats that were prevalent during that time.
One possible plotline could be that Negan initially tries to assert his dominance over Sam and Dean. He might see them as a threat to his own 'rule' in the world of the fanfiction. But Sam and Dean, being the hunters they are, would not take kindly to his posturing. They could end up in a battle of wits and strength. Another plotline could be that Negan has some information about a powerful supernatural entity that Sam and Dean are after. So they have to form an uneasy alliance. Sam might try to reason with Negan while Dean is more on the side of being cautious and ready to fight if needed.
One possible plotline could be Sam being captured by a villain and gagged to prevent him from using spells or calling for help. Another could involve a prank gone wrong within the Supernatural universe where Sam ends up gagged in a humorous yet tense situation. Maybe there's a plot where an otherworldly entity has the power to silence Sam by gagging him to stop his investigations.
One possible plotline could be Harry going on hunts with Sam and Dean. She might use her own unique skills, perhaps some magic - like in the Harry Potter world - to help them out in a way that surprises the brothers. Another could be her trying to fit into their world of hunting while also dealing with the typical sibling rivalries and protectiveness.
One interesting storyline could be the baby sister having special powers that she has to learn to control. Maybe she can communicate with spirits in a unique way, different from Sam and Dean, which adds a new dynamic to their hunting adventures.
In some fanfics, there could be a plotline centered around Bella's love life in the context of being Dean and Sam's sister. Maybe she falls for another hunter, and the brothers have to approve or disapprove. There could also be a plot where Bella is haunted by a past event that only she remembers, and she has to rely on her brothers to help her figure out what really happened and how to move on.
A lot of fanfictions also explore themes of sacrifice. Dean and Sam are often put in situations where they have to sacrifice something for each other or for the greater good. It shows their selfless nature. Also, the theme of redemption can be found in some, especially when they deal with characters who have made mistakes in the past.
Well, the theme of redemption is quite common. Whether it's Dean trying to make up for past mistakes or Sam finding his way back from a dark path. Love and friendship also play a role. Sometimes it's about their relationship with other hunters or people they meet. And of course, the struggle against fate. Many fanfics have them trying to break free from a pre - determined destiny.