I think it's because Nico is a character with a lot of depth. Fanfiction might delve too deep into his psyche and expose some very raw and disturbing aspects that people aren't prepared for. It could also be that the way some fanfiction writers portray his relationships, especially the more controversial ones, makes people scared to engage with it.
Maybe it's because Nico has some complex and often dark or angsty aspects to his character in the source material. Fanfiction might explore these in extreme ways that can be disturbing to some readers.
The plot could start with Naruto isolating himself. He would avoid his friends and stay away from the village. His friends would notice this change and try to reach out to him. But Naruto, being scared, would react badly. This would create a lot of confusion and worry among his friends. Then, as the story progresses, they would try to find out the reason behind his fear.
One reason is that it allows fans to fill in the gaps in the original story. Maybe the original source didn't fully explore their relationship, so fanfiction becomes a way to do that. Also, the idea of two different characters like Nico and Will coming together in a kiss is appealing as it combines different character traits and story arcs in an interesting way.
Another aspect is the source of his fear. It could be something magical, like a dark curse or an evil spirit that specifically targets him. Or it could be more emotional, like the fear of losing someone he cares about. When the author weaves these elements together well, creating a situation where Draco is truly scared, it makes for a great fanfiction. For instance, if there's a fanfic where Draco discovers a plot to harm his mother and he's powerless to stop it at first, his fear would be very believable and engaging for the reader.
One possible reason could be that Naruto has had some really bad experiences that made him lose his confidence. For example, if he faced a series of powerful enemies alone and got severely beaten each time, it might make him scared of everyone. Another reason could be some kind of curse or dark magic that affects his mind, making him fear those around him.
One of the main reasons is the rich mythology that the Percy Jackson world is based on. When you add the characters of Percy, Will, and Nico into fanfiction, it gives writers a chance to play with different mythological elements. Also, the characters themselves are very relatable. Nico's struggle with his identity, Will's determination to help others, and Percy's journey as a hero are things that many readers can identify with. This makes the fanfiction very appealing as it can explore these aspects in new and different ways.
The contrast in their personalities. Percy is more of a classic hero, while Nico has a darker, more brooding side. This contrast creates an interesting dynamic that fans like to explore in fanfiction.
The Percy Jackson universe itself is rich and full of possibilities. Fanfiction writers can play around with the mythology, the powers of the characters, and the different locations. For example, Nico's connection to the Underworld offers a whole new world to explore in fanfiction. It allows for creative and exciting stories that fans can't get enough of.
Strawhats fanfiction is popular because it offers different perspectives and possibilities. It can explore character relationships in more depth or introduce new elements that weren't in the original. Also, it gives fans a way to actively contribute to the fandom and share their creativity.
The unknown. When you don't know what the ghost will do next or what it really is, that's what makes it truly terrifying. For example, in a story where a person hears strange noises at night but can't see the source. It could be anything, and that sense of not knowing keeps the fear alive.