Well, perhaps the OC was a demon in their previous existence and in this reincarnation, they are born into a family of exorcists. They struggle with their new identity as they have the instincts of a demon but the heart of a human. Their reincarnation could be part of a grand plan to bridge the gap between the two worlds. They might have unique abilities that neither demons nor exorcists fully understand at first.
One idea could be that the OC was a powerful exorcist in their past life. In this reincarnation, they start off not knowing their true potential. As the story progresses, they begin to have strange dreams and visions that are actually memories from their past life. They then meet a character from their past life who helps them unlock their powers and they join the fight against demons.
First, think about their past life. What were they? An exorcist, a demon, or something else? This will shape their personality. If they were an exorcist, they might be more confident and brave in this life. Second, give them unique abilities related to their reincarnation. Maybe they can see the memories of their past self when they touch certain things. Third, create a backstory for their reincarnation. Were they brought back for a purpose? Maybe they are the key to stopping a great evil.
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.
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 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 plot idea could be that your OC discovers a hidden power within themselves that is connected to an ancient prophecy. They then have to figure out how to control this power while also dealing with the other exorcists who are either suspicious or jealous of them. Another idea is that a powerful demon has a special interest in your OC and is constantly trying to either turn them to the dark side or use them for some evil purpose. Your OC has to resist and find a way to defeat this demon.
One plot idea could be that the cat OC is a reincarnation of an ancient exorcist. It slowly starts to remember its past life as it encounters different demons and exorcists. This could lead to a journey of self - discovery and also help the main characters in their fight against stronger demons. Another idea is that the cat OC has a special bond with a powerful demon, but it has to keep this a secret from the exorcists. This creates a lot of internal conflict and also the potential for some tense moments when the secret is almost revealed.
One plot idea could be that the male OC is a new student at True Cross Academy. He's a bit of an outcast because of his mysterious past. As he tries to fit in, he discovers a hidden plot within the academy that involves demons. Another idea is that he has a special power that the exorcist order wants to use for their own purposes, but he has to decide whether to cooperate or not. A third plot could be that he is on a quest to find a lost artifact that has the power to either destroy all demons or give them ultimate power, and he has to face many challenges along the way.
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.
The OC could be a new student at True Cross Academy. They start off as an outsider but quickly show their talent as a tamer. This could create new friendships and rivalries with the existing characters like Rin and Yukio.
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.