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.
A cool fanfiction could be set in an alternate universe where the Supernatural world is known to everyone and the characters from The Raid are actually a special unit trained to deal with supernatural threats. They are called in to handle a situation where a powerful demon has taken over a large building, similar to the setting in The Raid. They have to use both their military tactics and some magic or supernatural weapons to defeat the demon.
One idea could be that members of Night Raid are transported to the Naruto universe. They team up with Naruto and his friends to fight a new, common enemy that combines the worst of both worlds' villains. Maybe they use their unique skills, like Akame's Murasame, in combination with Naruto's shadow clones.
A more complex plot could be that the Death Battle arena is somehow connected to their world, and Night Raid has to enter it to save one of their own. Inside the arena, they not only have to fight against the opponents in the battles but also deal with the strange rules and mechanics of the Death Battle world. This would put their teamwork and adaptability to the test.
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.
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.
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.
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.
In Supernatural, Sam and Dean Winchester are often popular in fanfiction. In the context of a crossover with The Raid, they could be the ones bringing in their knowledge of the supernatural world. From The Raid, Rama is a well - known character. Fanfiction might put him in a situation where he has to team up with the Winchesters. He could learn about the existence of demons and spirits from them and use his combat skills to help them in their hunts.
One plot idea could be Ketch getting involved in a time - travel adventure within the Supernatural universe. He could go back in time and interact with the Winchester brothers at different points in their lives, changing the course of events in unexpected ways.
A possible plot could involve the mystery spot being a place where the boundaries between the living and the dead are thin. A young girl, who has the ability to see ghosts, goes there to try and communicate with her deceased mother. But she soon realizes that there are malevolent spirits there too and she has to use her supernatural gift to protect herself and others who are drawn to the mystery spot's strange energy.