Well, one thing that could happen is that her relationships with other characters would shift. For instance, some male characters who were just friends might start having different feelings towards her. And she might face different challenges from other female characters who could be jealous or supportive in different ways.
A lot would change in terms of the story's themes. Instead of just being about Rin's journey as a male exorcist, it could become a story about breaking gender norms. She would have to deal with the expectations that society in the Blue Exorcist world has for both genders. It could also explore how her new - found identity affects her relationship with her demon heritage, as perhaps there are different implications for a female with a demon father compared to a male.
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.
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.
The story would change in terms of the romantic sub - plots. If Rin was a girl, there could be new love interests emerging. Maybe some characters who were just friends before would start to have romantic feelings for her. For example, Bon might see her in a completely different light and develop a crush.
Check out the official Blue Exorcist forums if there are any still active. Sometimes, fans post their fanfics directly on these forums for others to read and comment on. Also, don't forget to look at the comment sections of Blue Exorcist - related YouTube videos. Sometimes, fans will mention great fanfics they've read in the comments, and you might be able to track them down from there.
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.
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.
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.
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.
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'.