It could be an amateur writer who is a huge fan of One Piece and really likes Luffy. They might have come up with this idea to explore a different side of Luffy that isn't shown much in the original series.
Well, it could be about Luffy having a secret power that no one in the Straw Hat crew knows about. Maybe it's a power that he got from a mysterious encounter and he's keeping it a secret to protect his crew from potential danger.
Yes, there are quite a few. Some fanfictions might explore their relationship in the context of adventures where they share private moments during difficult situations. For example, during a battle when they are separated from the rest of the crew and have to rely on each other more closely.
One common theme is adventure - they might have their relationship develop during a special adventure that is different from the main story. Another could be trust, as they trust each other deeply in the original story, and in the fanfiction, this trust could be a stepping stone to a romantic relationship.
The idea of Percy having a secret girlfriend in fanfiction can open up a whole new world of character development. If his girlfriend is a new character introduced in the fanfiction, she could have unique powers or abilities that complement Percy's. Their secret relationship could be a way to explore Percy's more private and vulnerable side, away from his usual heroic adventures.
Yes, there are likely many fanfictions out there with the concept of Harry Potter having a secret wife. Fanfiction writers often come up with creative and alternative storylines to the original Harry Potter series. Some might explore this idea to add new dimensions to Harry's character or create a whole new plot within the wizarding world.
It could be Hinata. In the original series, Hinata has a deep affection for Naruto. In a fanfiction, it would be a natural progression for her to be his secret fiance. Maybe they got engaged secretly to avoid some political unrest in the village.
It might also be Gaara. Their initial encounters were filled with tension, but as the story could be twisted in fanfiction, that could develop into a different kind of relationship. Gaara's own journey of self - discovery and Naruto's influence on him could be a basis for a romantic relationship in fanfic.
The theme of acceptance is quite common. Fem Luffy often has to be accepted by the other characters in the story. This could be because she's different due to her gender or her slightly different personality traits. Also, the theme of self - discovery is prevalent. As fem Luffy navigates the pirate world, she discovers more about herself, her abilities, and her place in the grand scheme of things.