It could also be that he is influenced by the people around him. His friends or family in the fanfiction might encourage him to open his heart again. And this new relationship gives him a different perspective on love. The woman (or man) he decides to marry might have a unique charm that makes him believe in a new future together.
One reason could be that he has finally healed from his past traumas. Maybe he has found someone who truly understands his complex life as a former military man and friend of Sherlock. This new person might bring a sense of stability and normalcy that he has been longing for.
Another possibility is that he marries someone related to Sherlock's world. Maybe a detective or an investigator who he meets during one of Sherlock's cases. They start off as colleagues in solving the mystery and gradually develop feelings for each other.
One possible reason could be for a mission. Maybe in the fanfiction, there is a mission that requires Naruto to go undercover as a girl. This would allow him to access certain places or information that would be otherwise unavailable to him as a boy. Another reason might be for self - discovery. Naruto could be curious about what it's like to be a girl and decides to explore that side of himself in the fictional world.
One possible reason could be that Adrien is being blackmailed by Hawkmoth. Maybe Hawkmoth has some information or has threatened someone Adrien cares about deeply, like his friends or family. So, in order to protect them, he decides to help Hawkmoth, even though he knows it's wrong.
One common theme is the exploration of gender roles. Since John is female in these fictions, it often looks at how she navigates a world that was originally male - dominated in the Sherlock Holmes canon. Another theme is relationships, like her friendship with Sherlock or potential romantic relationships. Also, the theme of her proving herself in a male - dominated field, such as detective work, is quite prevalent.
One common theme is friendship. Writers often explore how an original character can form a deep and meaningful friendship with Watson. It could be a friendship that helps Watson through tough times or one that adds a new dimension to his life.
One common theme is the struggle with identity. As a vampire, John has to reconcile his new vampiric self with his old identity as a doctor and a friend. Another theme is the relationship with Sherlock. It often becomes more intense and complicated due to his vampiric nature.
One reason could be to protect the person he marries. If she's a commoner, for example, the noble society in Camelot might not accept their relationship. So, they keep it a secret. Another reason could be that the person he marries has some sort of power or identity that could put them in danger if it was known. For instance, if she was a sorceress, in Camelot where magic is feared, they'd have to marry secretly.
It allows for a different take on the Sherlock Holmes world. Since the OC is a new element, it can introduce fresh storylines that deviate from the original canon. For example, the OC could have special skills or a backstory that changes the dynamic between John and Sherlock. This makes for an exciting read as it combines the familiar with the new.
One common theme is adventure. Since John Watson is often associated with Sherlock Holmes and their detective work, the fanfictions might involve the reader going on adventures with him, solving mysteries. Another theme is romance, of course, as it's 'x reader' so there's usually a romantic element between the reader and John Watson.
One popular female John Watson fanfiction might be those that explore her in a modern-day Sherlock adaptation. For example, stories where she uses her medical skills in a different way, like working in a high - tech research facility while still being friends with a modern - day Sherlock. Another could be set in an alternate universe where she is a detective in her own right, solving cases parallel to Sherlock's and they end up collaborating in unexpected ways. There are also fanfictions that focus on her backstory, like her family life and how it shaped her into the person she is in the Sherlock - Holmes - inspired world.