Some fan novels might explore the backstories of the existing characters in more depth. For instance, we could learn more about Zoro's training with his old master in more detail. Maybe there are secrets about his training that were never revealed in the original series. This would give fans a better understanding of his character and his unwavering determination.
Perhaps a magical curse befalls the crew that causes several female characters in the One Piece world to be irresistibly drawn to the male members of the Straw Hats. This could lead to all sorts of chaos as the crew tries to deal with these new, unexpected relationships while still pursuing their main goals.
A plot could involve a supernova teaming up with an unexpected character, like a former villain who has a change of heart. They could go on a quest to find a long - lost treasure that has a special connection to the supernova's past. This would not only create an interesting dynamic but also explore the backstory of the supernova character more deeply.
Well, a plot could involve a vampire character who was once a regular pirate but got turned into a vampire. This vampire pirate then has to deal with his new cravings while still trying to achieve his original goals in the One Piece world, like finding the One Piece. And maybe there's a group of vampire hunters in the One Piece world who are after him.
One idea could be that Whitebeard is transported to a different time in the One Piece world. Maybe he ends up in the future and has to deal with new pirates and powers that he's never seen before. Another could be a crossover with another anime world, like he somehow gets stuck in the Naruto world and has to find his way back while dealing with ninjas.
One idea could be focusing on the children of the Straw Hat crew. For example, Luffy's son might inherit his rubber abilities but also have a unique power of his own. He could embark on a journey to find a new One Piece that is different from what his father was after, perhaps something more related to the forgotten history of the world. Another plot could be about the next generation of pirates from other crews teaming up with the Straw Hat descendants to face a new, powerful enemy that is even more menacing than the World Government in the original series.
Perhaps a villain from DBZ, like Frieza, finds a way to travel to the One Piece universe. He starts enslaving the pirates and taking over islands. Luffy and his crew would be the first to resist, but they quickly realize they need help. Goku and Vegeta then arrive and have to teach the pirates about ki and how to fight against Frieza's powerful minions. It would be a great opportunity to see the different fighting styles blend together.
One idea could be an OC Marine who discovers a long - lost Devil Fruit that has the power to control the tides. This could lead to a race among pirates and other Marines to either obtain or destroy it. Another could be an OC Marine who has a unique fighting style inspired by an ancient form of martial arts in the One Piece world. This Marine could then be sent on special missions to deal with powerful pirate threats.
One idea could be Mihawk getting transported to a different part of the One Piece world where he has to team up with the Straw Hats to face a new and powerful enemy. Maybe a group of ancient sea gods that are awakened and threatening to destroy all pirates. The Straw Hats could initially be wary of Mihawk but soon realize his skills are crucial for their survival.
One idea could be Ace crossing over into a modern - day world where he has to adapt to new technologies while still having his devil fruit powers. He might get involved in some sort of street racing using his fire powers to boost his vehicle.
A plot could revolve around Perona trying to cheer up Mihawk. Since he is often portrayed as a rather stoic and solitary figure. Perona could use her unique personality and ghostly antics to bring some unexpected humor and light into Mihawk's life. For example, she could create ghosts that re - enact funny scenes from Mihawk's past battles, which at first he is annoyed by but then starts to find amusing.