In a fanfiction where Naruto sealed away One Piece, it might have a huge impact on the characters. Luffy, for example, would be devastated as his life's goal is to find One Piece. He might even try to find Naruto to convince him to reverse it. The other Straw Hats would also be affected, their dreams of seeing the end of the Grand Line and the secrets One Piece holds vanishing. And the entire pirate community would be shaken to its core, as One Piece is like the ultimate symbol of their freedom and adventure.
One possible reason could be that his godlike powers were seen as a threat. The village elders or some powerful forces might have feared that Naruto, with his immense strength, could lose control and cause destruction. So, they decided to seal him away to protect the village and the world at large.
In a fanfiction, Naruto could have been taught a new and unique sealing technique by some powerful entity. This technique could be specifically designed to seal away powerful and mysterious items like One Piece. He might need to gather certain components or meet specific conditions in the One Piece world before he can perform the sealing. For example, he could need to be in a certain location, like the final island where One Piece is located, and use the natural energy or some special power source there to power his sealing jutsu.
The story could also focus on the efforts to unseal Naruto. His friends and the characters from the crossover world could embark on a dangerous journey to find the key or the magic to break the seal. In a Naruto - Harry Potter crossover, Hermione might use her vast knowledge of magic to research a way to free Naruto from the seal.
One possibility is that the story could focus on how the village copes without Naruto. Maybe there would be a different kind of unrest or a search for a new way to deal with the Nine - Tails' power. Without Naruto, characters like Iruka might have a different role, perhaps becoming more involved in trying to find a solution to the power void.
Well, perhaps in this fanfiction, the Avatar enters the Naruto universe and causes some sort of imbalance. Naruto, seeing the chaos it might bring, decides to seal away the Avatar. As time passes, Naruto starts to learn more about the Avatar's world and realizes that there are good things about the Avatar too. So he begins a journey to find a way to unseal the Avatar without causing more harm, all while facing enemies who want to use the situation for their own gain.
Maybe it's an ancient and powerful entity that has been awakened. Percy might be in the wrong place at the wrong time and this entity, seeing Percy as an intruder or a potential disruptor of its slumber, decides to seal him away using its otherworldly powers.
I'm not entirely sure as I haven't read that specific fanfic. But it might be about Naruto being sealed away in some way, perhaps in a different scenario from the original story. Maybe some new villain or power manages to seal him.
Well, it changes the power dynamic. With Naruto sealed away, the villages that once relied on his strength would be in a more vulnerable position. His friends and allies would also have to find new ways to protect their homes. It might lead to new alliances forming as different groups try to fill the power vacuum left by Naruto.
In a fanfiction where Naruto is a fishman in One Piece, Naruto would face new challenges. For example, dealing with the discrimination that some fishmen face in the One Piece world. But he'd also have new opportunities. He could use his fishman physiology to swim faster and deeper, discover hidden treasures, and maybe even find a way to connect the ninja world with the pirate world. His Nine - Tailed Fox chakra might also have some interesting interactions with the strange powers in One Piece.