One common theme is friendship evolving into something more. Since they start as friends and teammates, many fanfictions explore how their relationship deepens romantically. For example, they might realize their feelings during a training session or a competition.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of Free! fanfictions, including those about Rin x Haru. Just search for the relevant tags and you'll likely find many great stories.
Friendship is a very common theme. Writers often explore how the deep bond between Rin, Haru, and Makoto affects their lives. For example, they might write about how they support each other through tough times in school or in their swimming careers.
Swimming is also a big theme. Given that they are all swimmers, many stories will involve them training, competing, or having some kind of revelation related to their swimming careers. For instance, a story could be about how Rin's new training technique inspires Haru and Makoto to improve their own skills.
One common theme is the exploration of their deep emotional bond. Since they have a complex relationship in the original story, in the Yaoi fanfictions, this bond is often intensified and turned into a romantic or sexual relationship. For example, they might struggle with their feelings for each other while also dealing with external factors like their swimming careers.
One common theme could be care and concern. For example, if Rin is sick, Haru might show his worry in a very characteristic way, like being more protective. Another theme could be vulnerability. When sick, the usually strong - willed characters become more fragile, which can add an interesting dynamic to their relationship.
One common theme is the exploration of their deep emotional connection. Since they are often portrayed as having a strong bond in the original work, fanfiction may delve deeper into how that bond could develop into a romantic or more intimate relationship in a Yaoi context.
One common theme is the exploration of their relationship in a new, underwater world. Since they are mermen, it adds a different dynamic to their already complex bond.
One common theme is friendship. Many fanfics explore how their initial friendship blossoms into something deeper. Another theme is self - discovery. As they interact more, both Haru and Rin often discover new things about themselves.
One common theme is their friendship. You can find stories that explore how they became friends, the ups and downs in their friendship, and how they support each other through difficult times. For example, a story might focus on how Rin helps Haru get over his fear of change in the swimming world.
Good Haru x Rin fanfic often has well - developed characters. It stays true to the original personalities of Haru and Rin from the source material. For example, Haru's calm and sometimes aloof nature and Rin's passionate and determined side.