Another plot could involve Yukio traveling through time. Maybe he goes back to a time when the True Cross Order was first founded. He could witness the original exorcists and their methods, and bring that knowledge back to the present to improve the current exorcist training. This would also be a great opportunity to explore the history of the exorcist world in the Blue Exorcist universe.
One idea could be exploring the backstory of a minor character in the Blue Exorcist world. For example, you could write about the life of a lower - level demon that Rin and the others encounter. Another idea is a time - travel fanfiction where a character goes back to a crucial moment in the Blue Exorcist timeline and changes something, leading to a whole new set of events. Maybe a character gets sent to an alternate dimension where the rules of exorcism are different.
One plot idea could be that Naruto, with his immortality, somehow gets transported to the world of Blue Exorcist. He teams up with Rin and the others to fight a new kind of demon that is threatening both their worlds. His ninja skills combined with the exorcist powers would make for an exciting adventure.
Maybe a character from Blue Exorcist gets summoned into the Fate universe by mistake. They then have to adapt to the new rules and magic there, and form alliances with the Fate characters to find a way back home. Along the way, they discover new aspects of their own powers.
A third plot idea is that some of the characters from Blue Exorcist end up in the Naruto universe. Rin and his friends could be intrigued by the concept of ninjas and chakra. They might learn some ninja techniques and in turn, teach the ninja about exorcism. This could lead to a new generation of ninja - exorcists in the Naruto world. They could also help the ninja deal with a dark force that has some similarities to demons from their own world.
One idea could be a crossover where Naruto characters end up in the world of Blue Exorcist. For example, Naruto and his friends could be mistaken for exorcists and get involved in battles against demons. Another idea is to create an alternate universe where the main characters of Blue Exorcist have the powers of Naruto characters. Maybe Rin could have the Nine - Tails' chakra and have to learn to control it like Naruto did.
One idea could be that the main character is reincarnated as a powerful exorcist from a different era and has to adapt to the old ways of exorcising demons while still using their modern knowledge. Another could be a reincarnation where the character comes back as a demon with the memories of their past life as an exorcist, leading to an internal struggle.
One idea could be a character traveling back in time to prevent the initial conflict in the Arpeggio of Blue Steel world from starting. Maybe they go back to a time before the Fog first appeared and try to stop whatever caused it. This could change the entire course of the story, with new alliances and power dynamics emerging.
Well, for Blue Exorcist OC fanfiction, you could have a plot where your OC is a hybrid of a human and a demon. This causes a lot of internal conflict as they struggle with their identity and are feared by both sides. They try to find a place where they can belong. Another plot idea is that your OC is a master of a new form of exorcism that is not recognized by the True Cross Order. They have to fight to get their method accepted while also using it to combat a new wave of powerful demons. And then there's the idea of your OC getting caught in the middle of a power struggle between different factions within the exorcist world, having to choose a side while trying to stay true to their own beliefs.
One idea could be that the characters from Blue Exorcist are transported to the world of D Gray - Man. For example, Rin Okumura could meet Allen Walker. They might team up to fight a common enemy that is a mix of the demons from Blue Exorcist and the Akuma from D Gray - Man. Another plot could involve the exorcism techniques from both worlds being combined to defeat a super - powerful evil force.
One idea could be an OC tamer who has the ability to communicate with rare and powerful demons. For example, they might be able to understand the language of ancient demons that others can't. This would give them an edge in battles and in understanding the demon world better.