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.
Naruto would also bring his unique ninja skills. His ability to use chakra could be seen as a form of power similar to Devil Fruits or Haki in the One Piece world. For instance, his Rasengan could be a powerful attack that few in the One Piece world could defend against. And his Sage Mode would give him an edge in sensing and anticipating his opponents' moves.
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.
The Juubi could be a sort of ancient and powerful force that exists in both worlds in a hidden way. Maybe it was the origin of some of the devil fruits in One Piece. In Naruto, it's a known and feared entity. In the fanfiction, it could be that the Juubi's power was split long ago, and some of it ended up in the One Piece world, which is why there are such strange and powerful abilities there.
She'd probably fit in quite well. Her strong will and determination are similar to many characters in One Piece. She could use her ninja skills to make a name for herself in the pirate world. Maybe she'd become a sort of legend, like a new type of pirate with ninja abilities.
In this fanfiction, Naruto and Law's interaction could be complex. At first, their different personalities might cause some friction. Naruto is more open and impulsive, while Law is more reserved and calculating. However, when they see the potential in each other's abilities, they would start to cooperate. Naruto's positive attitude could gradually influence Law to be more open. They might have some friendly competitions, like Naruto showing off his Rasengan and Law demonstrating his Room ability. And in battles, they would learn to coordinate their attacks, with Naruto using his speed and agility and Law using his strategic spatial manipulation.
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.
Madara's powers would be both a boon and a bane in the One Piece fanfiction world. His ability to summon the Susanoo could be a fearsome sight on the Grand Line. It could be as intimidating as some of the giant sea kings or even the ancient weapons. On the other hand, his powers might not be fully understood by the characters in One Piece. For instance, the concept of chakra is different from Haki and Devil Fruits. But if written well, his presence could add a whole new layer of excitement and mystery to the story. Maybe he could form alliances with some of the Yonko or even oppose them, depending on the plot of the fanfiction.
He might fit in as a sort of super - strong outsider. Vegeta could be on a quest for his own kind of 'One Piece', which could be something related to power or a way to return to his own universe. His Saiyan abilities would make him a force to be reckoned with. For example, his ability to transform into Super Saiyan forms would be like a new level of power that no one in the One Piece world has seen before. He might start out as an antagonist but could end up teaming up with the Straw Hats if they had a common enemy.
They could fit in as a new type of pirate. The Rinnegan's powers would give them an edge in combat and exploration. For instance, they could use the Preta Path to absorb attacks from devil fruit users, like Logia types. This would make them a feared opponent on the high seas.
Sakura would fit in as a strong fighter. Her physical strength and combat training from being a ninja would make her a formidable opponent in the pirate - filled world of One Piece. She could hold her own in battles against pirates or other threats.