Well, it could be a story where Rin and Shiemi are on a mission together. As they face a powerful demon, Shiemi gets injured. Rin, worried sick, nurses her back to health. In a moment of relief and new - found closeness after the ordeal, they share a kiss. It's a sign of their growing bond and the start of a deeper relationship.
Perhaps in the fanfiction, they are at a school festival. There's a romantic atmosphere all around. Shiemi has been secretly in love with Rin for a long time. Rin, who has slowly realized his feelings for her, takes her to a quiet corner during the festival. He looks into her eyes and then they kiss, much to the delight of their friends who have been secretly shipping them.
It could end happily with them becoming a couple and continuing to fight demons together. They support each other in their exorcist duties and their relationship grows stronger over time.
In the Blue Exorcist series, Rin and Shiemi have a very special relationship. They are friends and comrades in the fight against demons. While the series doesn't really point towards them having a romantic future in an obvious way, they do care deeply for each other. Rin is a strong - willed and hot - tempered character, while Shiemi is more gentle and kind - hearted. Their personalities could potentially complement each other well in a relationship. However, they are both focused on their exorcist duties for now. So, it's hard to say if they would ever end up together in that sense.
One common theme is the growth of their relationship. Since Rin is a demon - human hybrid and Shiemi has her own insecurities, their relationship development is often explored. For example, how they learn to trust each other more deeply.
In the fanfiction, their kiss could be a turning point in their relationship. It might symbolize a deeper connection between them. It could occur in a beautiful, perhaps a secret garden within the academy. The description of the kiss could involve the softness of their lips, the quickening of their heartbeats, and the surrounding scenery enhancing the romantic atmosphere.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of Blue Exorcist fanfics. Just type 'Blue Exorcist Rin x Shiemi' in the search bar and you'll get numerous results. Some might be focused on their relationship development, while others could be adventure - themed with their relationship as a side plot.
One possible plot could be that Rin's demonic powers go haywire during a difficult exorcism. As a result, his body starts to regress into a baby form. His friends at True Cross Academy would be super confused. Yukio might initially think it's some kind of new demonic curse. They would then have to figure out how to reverse the process while also protecting baby Rin from other demons who might see him as an easy target.
It could be about Naruto somehow ending up in the world of Blue Exorcist and taking on the role or having the abilities of Rin. Maybe he has to fight demons in the Blue Exorcist style while still having his Naruto - like skills.
In the Blue Exorcist story, Rin and Shiemi's relationship has the potential to develop further. They have a certain chemistry that is evident in their interactions. Rin's brashness and Shiemi's calmness can create an interesting dynamic. But the story is full of other elements that could either push them together or keep them apart. For example, the threat of demons and the secrets within the exorcist community. Also, their own personal insecurities might hold them back. Overall, it's hard to put a number on the chances, but it's not impossible for their relationship to evolve.
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'.