Another concept could be that the Nine - Tailed Fox from Naruto somehow gets loose in the One Piece world and its chakra affects Devil Fruits, causing strange mutations. Luffy and his crew then have to team up with Naruto and his friends to find a way to stop the chaos.
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.
A good fanfiction also needs a strong plot. You could start with a conflict that involves all three aspects. Say, a powerful enemy that is a combination of the threats in Naruto and One Piece is threatening the New World. Develop the relationships between the characters from both series. Let them learn from each other and grow. For instance, Zoro could teach Sasuke some new sword techniques, while Naruto could inspire Luffy with his never - give - up attitude.
If Naruto were in the One Piece world, he could form interesting alliances. He might team up with Luffy and the Straw Hats. Naruto's ability to make friends easily would fit right in with the Straw Hat crew's spirit. His Rasengan could be a very powerful attack against some of the strong foes in the One Piece world. Also, his experience with dealing with different types of enemies in his own world would give him an edge in strategizing in the new and strange world of One Piece.
Naruto's ninjutsu skills could be very powerful in the One Piece world. His Rasengan, for instance, is a concentrated ball of chakra. In the One Piece world, it could be as strong as some of the more powerful Devil Fruit attacks. His ability to run up walls and trees using chakra control could also be useful in the more diverse terrains of the One Piece world, like running up the side of a ship or a rocky island.
Naruto could fit in by using his physical abilities. His superhuman strength, speed, and endurance, which he developed through his ninja training, would make him a formidable opponent in the One Piece world. He could easily take on weaker pirates or even some of the lower - level Marines.
Kankuro could use his puppet - making skills to his advantage. He could create puppets that are resistant to the elements in the One Piece world, like water - proof puppets since there are a lot of sea - related challenges there. Also, he could make puppets that can fly to deal with the various islands and different terrains.
Naruto's ability to create shadow clones would be extremely useful in the One Piece world. He could send clones to gather information across different islands while he and the Straw Hats are on their main journey. His Rasengan, being a powerful concentrated attack, could be used against tough enemies like the Yonko's commanders. And his immortality means he can keep using these abilities without the fear of death, making him a very formidable ally in the One Piece universe.
Naruto's Rasengan could be seen as a super - powerful attack similar to some of the advanced Haki techniques in One Piece. His ability to create shadow clones would be a huge advantage in battles and also for things like scouting or distractions. His chakra control could potentially be related to Haki control as well. And his immortality would make him a very unique and feared presence in the One Piece world, as most characters there are not immortal.
A frequent theme is the exploration of how characters from Naruto would fit into the One Piece world. This includes how they would interact with the diverse races and cultures in One Piece. Also, the idea of teamwork across the two universes is quite common, as both Naruto and One Piece emphasize the importance of strong bonds between characters. For example, we might see Sasuke teaming up with Zoro to take on a powerful enemy. There's also the theme of discovery, as Naruto characters get to experience the strange and wonderful things in the One Piece world like Sky Island or Fish - Man Island.