If Naruto sealed away One Piece in a fanfiction, it could lead to some really interesting crossovers. Naruto's powers are very different from those in the One Piece world. His sealing jutsu might be something that no one in One Piece has ever seen before. This could attract the attention of powerful characters in One Piece, like the Yonko or the Admirals. They might try to study Naruto's power or even force him to unseal One Piece. It could also create a new dynamic where Naruto has to deal with the very different values and cultures of the One Piece world while being hunted for what he's done.
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.
In such a fanfiction, the Fairy Tail guild might be thrown into chaos. They could initially be confused about this strange event from another world. Some members might see Naruto's sealing as a threat to their world's balance, while others might be more curious and try to find a way to break the seal.
One possible reason could be a misjudgment by the Hokage. Maybe they believe Naruto's power is too dangerous and uncontrollable at a certain point, so they decide to seal him away to protect the village. For example, if Naruto's Nine - Tails' power starts to go berserk and they see no other way to stop it without harming him too much.
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.
Well, Naruto might gain some new powers or knowledge from having Tobi sealed. Maybe he could access Tobi's memories and use that information to prevent future threats.
In some fanfictions, if Naruto was sealed in the Shinju, it could lead to a major shift in the power balance. His friends might go on a desperate quest to free him. The ninja world could be plunged into chaos as his absence leaves a huge void in terms of power and leadership.
In such a fanfiction scenario, many things could occur. One possibility is that the world would be thrown into chaos as Naruto is a major force for stability. His friends might embark on a dangerous mission to rescue him. They would have to figure out the mechanism of the seal on the moon and find a way to break it. It could also lead to power struggles on Earth as various factions try to take advantage of Naruto's absence.
One possibility in such a fanfiction scenario is that Kushina's own chakra, which was already special as an Uzumaki, could have an impact on Naruto while he's sealed within her. It could either strengthen him in some ways or cause unforeseen consequences. Also, characters like Minato would have a different relationship with the situation. Instead of seeing Naruto as his son on the outside, he might be more focused on the presence of Naruto within Kushina, which could change how he approaches protecting his family and the village.
Well, he might start by being really confused. The Devil Fruits would be so strange to him compared to chakra. He could probably make friends with Luffy easily though, since they both have that adventurous spirit. And his ninja skills could come in handy when fighting pirates. Maybe he'd teach some of the Straw Hats some cool ninja moves.
In a fanfiction with Naruto having Frieza sealed in him, there would be a clash of cultures too. Naruto comes from a world of ninjas with honor and teamwork, while Frieza is a tyrant from a universe where power rules all. Naruto would have to find a way to balance Frieza's power without losing his own values. His training would take a whole new turn as he tries to master Frieza's power. And there could be a lot of internal conflict as Naruto battles with Frieza's consciousness inside him, trying not to let Frieza take over his body.
Well, he might gain some spooky powers. Maybe he could use the Shinigami's abilities to sense death or even communicate with the dead. It could also put him in a really dangerous situation, as the Shinigami's power might be too much for him to control at first.