One interesting headcanon could be about the backstories of the Servants. For example, there might be a headcanon that a certain Servant had a secret friendship in their past life that influenced their character in the game. Another could be related to the magic system in the Fate Grand Order universe, like a unique way of casting spells that isn't shown in the main story.
Well, a popular headcanon is that Percy can see the memories of the ocean. So in fanfiction, he might touch an old piece of coral and suddenly be flooded with images of ancient sea battles or long - lost civilizations. Also, some think that Annabeth has a fear of heights that she hides really well because she doesn't want to seem weak. As for Grover, perhaps he has a family of satyrs in a hidden grove that he secretly visits and they tell him old satyr tales that he then uses to his advantage on quests.
I like the headcanon where Annabeth is a master of disguise. In fanfiction, she can blend into any crowd, not just because of her intelligence but also because she has some magical items that help her change her appearance. It makes her even more resourceful on quests. Grover, on the other hand, has a headcanon where he can grow plants with just a touch. So if they're in a barren area, he can create a little oasis for them to rest in. This shows his connection to nature in a more powerful way.
One idea could be that Neville Longbottom is secretly a powerful wizard all along but has been suppressing his power due to self - doubt. Another could be that the Hogwarts portraits have their own little society and gossip network within the castle walls. And a third one might be that Hagrid has a secret family of magical creatures in the Forbidden Forest that he takes care of.
It can add more depth to minor characters. For example, by creating headcanons about characters like Lavender Brown, we get to see a side of the Hogwarts student body that wasn't fully explored in the original. It gives them more personality and backstory.
One common plotline could be the OC being a new addition to Toman and catching Sano Manjiro's eye. Maybe they start off as friends, helping each other out in gang fights or dealing with the problems within the gang world. Another could be a more romantic plot where the OC has some sort of special ability or knowledge that Manjiro finds fascinating, leading to a deeper connection between them.
Involve the OC in the world of Sano Manjiro in a natural way. Don't just have her appear out of nowhere. For instance, if she's a new student at their school, show how she gradually gets to know the members of Toman. Let her have her own relationships with other characters besides Manjiro. This way, she becomes a more integral part of the story and seems more believable.
This book was called "Conan: I'm Really Not a Tokyo Ghost Tale." The characters in the book included: male protagonist Sano Shinjiro, male protagonist Kurosawa, male protagonist Kurosawa, male protagonist Kurosawa Claw, male protagonist Kurosawa Yu, male protagonist Amuro, male protagonist Conan, male protagonist Gin, female protagonist Mao Lilan, female protagonist Suzuki Sonoko. This book can be read on Qidian Chinese website.
"Conan: I'm Really Not a Tokyo Ghost Story" Author: Flying in Jianghu is a light novel/derivative doujin novel with elements of transmigration, system style, invincible style, scheming, and comics.
[User recommendation: Daily Brewery, the protagonist should be a fake bar!] Conan Doujinshi is the one that suits my taste the most.
I hope you will like this book.