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.
One interesting storyline could be a character from Naruto who has the power of the Juubi getting transported to the One Piece world and ending up in a harem situation. For example, Naruto himself with the Juubi's power might land on an island in One Piece and attract a group of female characters due to his unique abilities and kind heart. They would then embark on adventures together, with Naruto's ninja skills and the Juubi - influenced powers being a new element in the One Piece universe.
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.
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.
Yes, there are likely some. The Naruto and One Piece universes are so vast that when combined with the concept of a godlike Juubi in fanfiction, it can create really interesting and unique stories. Some fanfictions might focus on a character having Juubi - like powers in the One Piece world, or Naruto characters interacting with One Piece characters in a scenario involving the Juubi. However, the quality can vary greatly depending on the writer.
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.
You can start by looking on popular fanfiction websites such as FanFiction.net or Archive of Our Own (AO3). These platforms have a large collection of fanfictions from various fandoms, including Naruto and One Piece crossovers. Just use the search function and type in relevant keywords like 'godlike juubi naruto one piece' to see if there are any available stories.
Common themes in 'naruto one piece harem fanfiction' include power struggles. Characters from Naruto and One Piece have different levels of power, and in the harem setting, there can be competition for dominance. Also, cultural clashes are common. The two universes have distinct cultures, and when the characters come together in a harem, these differences create interesting storylines. For example, the ninja way from Naruto and the pirate code from One Piece can sometimes clash or be reconciled. Moreover, love triangles or polygons are also typical, as the harem concept implies multiple romantic interests for the main characters.
One possible interesting plot combination could be that Naruto, with his Juubi powers, somehow gets transported to the One Piece world. He might team up with Luffy and his crew. Naruto could use his shadow clone jutsu to help in battles against the pirates or the Marines. His ability to sense chakra could also be used to detect hidden treasures or enemies with unique powers in the One Piece world.
Obviously Naruto would be the main character. As for the harem, characters like Hinata could be part of it. She has always had feelings for Naruto. Sakura might also be involved as she has been an important part of Naruto's life. And perhaps some new female characters could be introduced, like a powerful kunoichi from a far - off land who senses Naruto's new power from the Juubi and is drawn to him.