Naruto's Rasengan could be used in a similar way as some of the physical attacks in One Piece. For example, it could be as powerful as Luffy's Gear attacks. Also, his ability to jump great heights and distances might be on par with some of the Zoan Devil Fruit users' enhanced physical abilities.
Naruto's transformation abilities, like the Nine - Tails transformation, could be compared to the Zoan type Devil Fruits in One Piece. However, his transformations are more based on chakra control. In a fight, he could use his transformations to overpower opponents. Also, his ability to communicate with animals, which he has shown in the Naruto series, could potentially be used to communicate with some of the strange creatures in the One Piece world, giving him an edge in certain situations.
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.
Well, Naruto's chakra - based abilities would be quite unique in the One Piece world. His Rasengan, for instance, would be a powerful new type of attack that most characters in One Piece haven't seen. His ability to walk on walls and water using chakra would also give him an edge in many situations. He might also be able to use his chakra to sense things in a different way than the Haki users in One Piece, which could be a big advantage when exploring islands or looking for hidden treasures.
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.
In these fanfictions, the different universes blend in various ways. The Juubi can act as a sort of catalyst. It might be that the Juubi's power is so great that it creates a portal between the Naruto and One Piece worlds. Once the characters start interacting, they share their knowledge and skills. The One Piece characters learn about ninjutsu and the Naruto characters learn about sailing and the different islands. In the harem aspect, the main character's journey through both worlds attracts different female characters from each, creating a diverse harem that represents both universes.
Naruto would probably be very curious about Nami's world. He might constantly ask her questions about the sea, the different islands, and the pirates. Nami would be initially cautious but then grow to like Naruto's enthusiasm. She could use Naruto's strength to her advantage in dangerous situations, like when facing powerful enemies on the high seas.
Another possibility is that the One Piece world has its own version of the Juubi. The poneglyphs could hold secrets about this powerful entity. A character like Robin, with her ability to read the poneglyphs, discovers information about the 'One Piece Juubi'. Then the story could follow how the different pirate crews react to this new threat or opportunity, and how they might interact with Naruto characters if they were to cross over.
Naruto would probably be really curious about the Devil Fruits in the One Piece world. The female characters like Nami might be initially cautious of him but then intrigued by his ninjutsu. For example, when Naruto uses his Rasengan, they would be amazed. And Naruto would be in awe of the Grand Line and all the strange islands, while the girls would show him around.
His chakra powers might be affected by the different energy in the One Piece world. Maybe his Rasengan would have a different look or effect. And his ability to sense chakra could turn into something like sensing the presence of Devil Fruits or Haki. His shadow clones could be used in new and creative ways, like scouting in the vast ocean or exploring uncharted islands.
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.