Well, sometimes in these fanfictions, the Juubi's power might cause some sort of distortion in the One Piece world's power balance. And in the harem aspect, it could be that the female characters are attracted to the mystery and strength that comes with the Juubi - related powers. Maybe a character like Luffy in One Piece gets a taste of the Juubi's power and it changes his perception of strength, and as a result, he gathers a harem of female pirates or islanders who are intrigued by this new, more powerful Luffy.
A possible storyline is that the Juubi's essence somehow merges with a Devil Fruit in the One Piece world. This creates a new and extremely powerful item. In the harem part, the female characters from both Naruto and One Piece universes are drawn to the person who possesses this new entity. They all have different motives, some might be after power, some might be genuinely attracted to the person's character. And as they come together, there are all sorts of interactions, like the Naruto characters teaching the One Piece characters about chakra, while the One Piece characters introduce their own unique aspects like the Grand Line and different races.
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.
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.
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.
One idea could be that a character from Naruto, like Naruto himself, somehow gets transported to the One Piece world. In this new world, he attracts a harem of various female characters from both Naruto and One Piece universes. They all become intrigued by his unique abilities and his positive attitude.
A cool plot might be that a mysterious figure uses the concept of a harem to manipulate Naruto in the context of the Juubi. The harem members could be from different villages and have different abilities related to the Juubi's energy. Naruto has to figure out who is behind this manipulation while also dealing with his growing feelings for the members of the harem. He could discover that there is a deeper connection between the Juubi and the creation of this harem situation, and that by solving this mystery, he can prevent a greater disaster.
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.
One interesting fanfiction could be where Naruto has godlike powers and teams up with Luffy from One Piece to fight a common super - powerful enemy. They combine their unique skills, like Naruto's Rasengan and Luffy's Gear Fourth, in epic battles across the combined Naruto - One Piece universe.
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.
One popular storyline could be Naruto and Hinata having to face the Juubi together. In this scenario, Naruto's power and Hinata's supportiveness combine. For example, Naruto might be in the front lines fighting the Juubi's brute force while Hinata uses her Byakugan to detect any weak points or incoming attacks from the Juubi. Their love for each other could also be a factor, with Hinata's unwavering belief in Naruto giving him the extra boost he needs to keep going. Another storyline could involve the Juubi somehow affecting Naruto and Hinata's relationship. Maybe the Juubi's presence or its chakra has an influence on Naruto's mind, and Hinata has to fight to bring him back to his true self. This could lead to some really emotional and intense moments between them as Hinata tries to break through whatever hold the Juubi has on Naruto.
One interesting storyline could be a new student arriving at One Piece Academy with a mysterious power that no one has seen before. This new character might disrupt the existing power balance among the students and lead to various challenges and friendships being tested.