Well, perhaps in the fanfic, the fever is not just a normal illness. It could be a result of some sort of curse or dark magic aimed at Rin. His brother Yukio might be in a panic as he tries to figure out what's wrong with Rin and how to cure him. Meanwhile, Rin is in a weakened state and has to fight off some minor demons that sense his vulnerability.
In this fanfic, it could start with Rin waking up with a fever. He initially brushes it off as just being a bit under the weather. But as the day progresses, his body temperature rises and he starts to lose control of his demonic powers a bit. His friends like Shiemi and Izumo notice his strange behavior. They try to help him, but they also face challenges as Rin's feverish state makes it difficult for him to communicate clearly, and they have to figure out how to keep him safe and get him well.
His friends would be very worried. Shiemi, for example, would probably bring some herbs that she thinks might help with the fever. She would be gentle and try to make Rin comfortable. Izumo, on the other hand, would be a bit more brusque but still concerned. She might scold Rin for not taking better care of himself while also trying to find a solution to his fever.
Perhaps in the fanfic, Rin's death could be a result of a curse. He might be tricked into a situation where he's cursed, and no matter what he does, the curse slowly drains his life. As he nears death, he could be reflecting on his relationships, his journey as an exorcist, and all the things he didn't get to do. His friends would then be on a desperate quest to find a way to break the curse, but they're too late.
It could be about Rin being bound to a powerful demon or exorcist as a familiar. Maybe he has to follow their commands and use his powers in ways he didn't expect. For example, he might be forced to fight other exorcists or demons on behalf of his 'master'.
In 'Blue Exorcist Rin fanfic', a very frequent theme is Rin's battle against his inner demons, both literally and figuratively. His journey to master his demonic powers while still remaining true to his human side is often explored. Also, his relationship with the exorcist community, whether it's facing discrimination or finding acceptance among them, is a common theme. Additionally, his friendships, especially with the people he meets at True Cross Academy, are often a big part of the stories.
One possibility could be that his relationships with other characters would change drastically. For example, his friendship with Yukio might become more protective on Yukio's part as he now has a sister - like figure in Rin. Also, his interactions with the other exorcist trainees could be different. They might be more hesitant around him as a girl, or some might be overly friendly in a different way.
One great fanfic could be 'The Bond Beyond Worlds'. It delves deep into the relationship between Amaimon and Rin. The author beautifully portrays how their unique connection evolves over time, with Amaimon's mischievous nature clashing and yet also complementing Rin's more determined and righteous personality. It's filled with exciting adventures and some really heart - warming moments that fans of the pairing will surely enjoy.
In a fanfic where Rin is secretly a girl, the author could explore themes of identity and self - discovery. Rin might struggle with coming to terms with her true gender while also dealing with the challenges of being an exorcist. It could add a whole new layer of depth to the character and make for a very engaging story.
Some fanfics focus on Rin's power development in a different way from the original story. Maybe she discovers new powers or has to learn to control her existing ones in a more challenging situation. There are also fanfics that explore Rin's family relationships in more depth, especially her connection to her father. This could involve flashbacks or new revelations about her family's past.
The plot might involve Rin using his skills as an exorcist to deal with the challenges of being a tamer. He could encounter various types of creatures that need taming. Since he has the power of Satan within him, it could add an interesting twist. He might struggle to control his inner demon while trying to tame other beings, and this internal conflict could drive the story forward.
One popular Rin x Izumo fanfic is 'The Unspoken Bond'. It delves deep into their relationship, exploring how their different personalities interact. The story shows how Rin's brashness and Izumo's more reserved nature create an interesting dynamic. Another good one is 'Under the Cherry Blossoms', which is set in a beautiful, almost dream - like world where their feelings for each other start to blossom.