In fanfiction, this character could be a key to uniting different pirate crews. With the power of the Rinnegan, they could communicate telepathically with crew members, like the Animal Path allows in Naruto. This would help in coordinating attacks and strategies during battles against the Marines or other pirate crews. Also, the Deva Path's ability to control gravity could be used to create new islands or sink enemy ships, changing the geography of the One Piece world and having a huge impact on the power balance.
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.
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'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.
He would likely become more brooding. With the dark and godlike Rinnegan, Naruto might start to question his previous beliefs. He could be more isolated as he tries to figure out his new power, which is different from the chakra he's used to. His usual sunny and optimistic personality might be overshadowed by a more serious and cautious side.
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.
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.
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.
Sasuke's character could potentially bring a new level of intensity to the One Piece world. His Sharingan abilities, like seeing through illusions and predicting movements, might make him a formidable opponent. He could be a lone wolf type of character in the Straw Hat crew, initially aloof but gradually warming up to the others, much like Zoro in some ways.
Another plot could involve a battle between a One Piece character who has obtained the Godlike Rinnegan powers. This character might use the Rinnegan abilities like the Six Paths techniques to fight against the powerful foes in the One Piece universe, like the Yonko. They could also use the power to control the tides or summon powerful sea creatures, creating a whole new dynamic in the battles.
The Rinnegan's ability to manipulate gravity (Deva Path) could be used in the One Piece universe in interesting ways. For instance, a character with the Rinnegan could create artificial islands or even reverse the flow of waterfalls on certain islands. This would be a huge advantage in a world where controlling the environment can be crucial in battles and exploration.