Maybe a fanfiction where a bagel shop in the Supernatural universe is a front for a secret society of hunters. The bagels there are made with ingredients that can ward off demons, and the story could revolve around the main characters uncovering this mystery.
First, you should have a basic understanding of the Supernatural universe, its characters, and the rules. Then, think about how the bagel will fit into this world. Will it be a normal food item with a strange backstory, or something more magical? Start with a simple plot idea, like the characters stopping at a diner and the bagel being the key to a mystery they're about to solve.
One idea could be exploring the relationship between the characters in more depth. For example, focusing on how Sam and Dean's bond was tested during that episode. Another idea is to create an alternate ending where things took a different turn.
One idea could be exploring the backstories of the different bloodlines in more detail. For example, you could focus on a particular family within a bloodline and how they've been affected by the supernatural elements over generations. Another idea is to create a new character who is a hybrid of two different bloodlines and show their struggle to fit in or their unique powers.
One idea could be a crossover where Goku from DBZ ends up in the Supernatural universe and has to team up with the Winchester brothers to fight demons. He could use his ki blasts against the hordes of hell.
One idea could be a character suddenly gaining supernatural powers and having to learn to control them while also dealing with the regular Supernatural threats. For example, a hunter who gets the power to see the future and has to figure out how to use it to prevent disasters in the hunting world.
Perhaps a fanfiction where a character from The Raid has some sort of special ability like the characters in Supernatural. Maybe he can see ghosts or has some sort of psychic power. This new ability could help him during the Raid and also change his relationship with his team members. He might be seen as an outcast at first but later prove to be very valuable.
One idea could be a story where a group of supernatural law enforcers have to stop a powerful vampire coven from taking over a city. They could use their unique powers like shape - shifting or magic spells to combat the vampires.
Well, you could have a fanfiction where a new character creates a chat room specifically for Supernatural characters to interact with each other outside of their usual battles. It could be a place where they share their fears and hopes. Maybe Castiel accidentally enters this chat room not knowing what it is and has some really funny exchanges with the other characters. Also, the chat room could be a time - traveling one where characters from different eras of the Supernatural universe can communicate.
One idea could be that the Winchester brothers are on a hunt in a small town where Hannibal Lecter has set up his practice. They initially suspect him of being a monster, but as they dig deeper, they realize he's a different kind of evil. Another could be that Hannibal gets intrigued by the existence of demons and angels in the Supernatural universe and tries to study them. He might even team up with the Winchesters in an uneasy alliance to take down a greater threat.
One idea could be a new character arriving at the rebel camp with a mysterious power that both intrigues and scares the existing members. For example, they might be able to control the elements in a way that's different from any known magic in the Supernatural world.