First, do your research on both Overwatch and mermaid lore. Know the characters in Overwatch and the typical traits of mermaids. Then, create an interesting plot. Maybe the mermaid reader could be a new hero or a key to solving a problem in the Overwatch world. Develop your characters well, giving the mermaid reader unique personality traits. For example, she could be shy or bold. Also, describe the settings vividly, whether it's the underwater mermaid kingdom or the Overwatch base.
One common theme is the idea of redemption. Genji, with his past full of mistakes and seeking to atone, and Tracer, who represents hope and a positive future, can often be part of a story where Genji finds more redemption through his relationship with Tracer.
One common theme is family and reconciliation. Since they are brothers, many fanfics focus on how they can overcome their past and reunite. Another theme is redemption. Hanzo, in particular, is often depicted on a path to redeem himself for what he did to Genji. Also, honor and the Shimada clan's legacy play a role as they both are part of this important clan.
Tracer is quite popular. Her upbeat and positive personality makes for interesting interactions with the reader. For instance, a fanfic could be about the reader time - traveling with Tracer and experiencing different events in the Overwatch timeline.
A lot of these fanfictions have a theme of protection. Reaper, despite his menacing appearance, protecting the reader from various threats in the Overwatch universe. There's also the theme of discovery, where the reader gets to discover the real person behind Reaper's intimidating facade, and as they do, a relationship begins to form between them. It's often about peeling back the layers of his character.
One common theme is redemption. McCree and Hanzo both have complex pasts, and many fanfictions explore how they find redemption through their relationship. Another theme is trust. Since they come from different backgrounds in the Overwatch world, building trust is a big part of their stories.
The 'bnha x overwatch fanfic' is interesting as it gives a chance to see beloved characters in new situations. We get to see how heroes like Reinhardt from Overwatch would fare in the world of bnha with all its quirks and hero schools. It also opens up possibilities for crossovers that we would never see in the official works of either franchise. Fanfic writers can take liberties and create unique story arcs that blend the best elements of both bnha and Overwatch.
One common theme is the healer - patient relationship. Since Mercy is a healer in Overwatch, often the OC is in need of her medical assistance, which then leads to a deeper connection. Another theme is the exploration of Mercy's angelic nature and how the OC relates to it, like being inspired by her or having a spiritual connection.
One great Overwatch x Mermaid Reader fanfic is 'Under the Sea and Overwatch'. It has an interesting plot where the mermaid reader character gets involved with the Overwatch heroes in a unique underwater mission. The author does a great job of describing the mermaid's world and how it intersects with the Overwatch universe.
The appeal lies in their complex relationship. They were once comrades in Overwatch, and now they're on opposite sides. Fanfictions can explore how they might still have some loyalty or feelings for each other.