One reason could be a devil fruit power gone wrong. Maybe he ate a fruit that was supposed to give him some new ability but had an unexpected side effect of changing his appearance to that of a girl. Another possibility is that he was hit by some strange magic or curse during an adventure on an island. It could also be the result of a prank by some powerful and mischievous character who has the ability to change appearances.
One possible reason could be a spell gone wrong. Maybe some dark magic was accidentally unleashed that started to change Harry's appearance to resemble Snape's. Another reason might be a side - effect of a powerful potion he was forced to drink in the fanfiction's plot.
Maybe in that fanfiction, Nami's character is distorted in a way that Luffy, with his straightforward and honest nature, can't stand. For example, if she is portrayed as overly greedy or mean in a way that goes against the essence of her character in the original work.
One possibility is that he's trying to prove a point. Maybe there's some discrimination or unfair treatment towards girls in the wizarding world that he wants to highlight. By dressing like a girl, he can put himself in their shoes, so to speak. Another reason could be that he's influenced by a new friendship or relationship. He might have a close female friend who has inspired him to try dressing like a girl, either as a form of bonding or because he admires her style.
In some fanfictions, character corruption can be a factor. The author might decide to corrupt the pure relationships in the original story. So Ace and Sabo are written as hating Luffy because they are influenced by some external dark force or bad ideology within the fan - created universe.
It could be really interesting! Maybe the characters from both shows could cross paths in a mysterious town event.
Well, a crossover could involve a mystery that brings the two groups of characters together. They could combine their unique skills and personalities to solve it. Or, it could be a story about friendship and romance across the shows' worlds.
It could be just for the fun of it. Writers often like to play with the established characters and create unique scenarios. In this case, seeing strong Naruto in a girl - like form can lead to humorous situations, like when his friends first see him and their shocked reactions. Also, it can open up new plotlines, such as Naruto having to disguise himself as a girl for a mission and having to navigate the situation while still being his strong self.
The shared ideology might be another factor. Both Luffy and Dragon seem to oppose the unjust systems in the One Piece world. Luffy wants to bring freedom to the islands he visits, while Dragon aims to topple the World Government which is often seen as corrupt. So in fanfiction, this common ground can be exaggerated to make Luffy more like Dragon.
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 reason could be as a form of punishment. Maybe Percy did something that Artemis considered disrespectful to the female gender or to her values as a goddess. Another reason might be for a mission. Artemis could think that in a female form, Percy would be able to infiltrate a place or situation that is only accessible to females.