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.
Maybe in the fanfiction, the One Piece characters could gain some of the abilities from Naruto. For example, Luffy could learn how to use Rasengan. And the Hokage characters would have to adapt to the strange rules of the Grand Line in One Piece, like the different sea currents and islands with their own unique ecosystems.
Another concept could be a story where a ninja from the Naruto world gets transported to the One Piece universe and has to deal with pirates while also having the Sharingan. This ninja could use the Sharingan to copy the fighting styles of the powerful pirates, like Zoro's swordsmanship or Sanji's kicks, and incorporate them into their own fighting style.
Another concept could be that the Nine - Tailed Fox from Naruto somehow gets loose in the One Piece world and its chakra affects Devil Fruits, causing strange mutations. Luffy and his crew then have to team up with Naruto and his friends to find a way to stop the chaos.
Another possibility is a story where Nami discovers a strange object on an island that transports her to the Naruto universe. There, she meets Naruto and they learn from each other. Nami could teach Naruto about the value of money and trading in the pirate world, while Naruto could show Nami how to master her own inner strength and use it to protect her friends more effectively. This could lead to an interesting adventure as Nami tries to find a way back to her own world with Naruto's help.
One idea could be a crossover where Naruto somehow ends up in the One Piece world and meets Nami. He could use his ninja skills to help Nami on a treasure hunt. Maybe he uses his shadow clone jutsu to distract enemies while Nami figures out the location of a hidden treasure using her navigation skills.
One idea could be a crossover where Naruto's world is invaded by the Gem Homeworld from Steven Universe, and the Juubi's power somehow gets involved in the conflict. For example, Naruto and his friends might have to team up with Steven and the Crystal Gems to fight off the combined threats.
One idea could be a character from the One Piece world somehow obtaining the Rinnegan. This character could then use the Rinnegan's powers, like the ability to see chakra, in the context of the One Piece world. For example, they might be able to detect the 'life energy' of Devil Fruits or sense the hidden strength of characters like Luffy's Gear forms. It would create a whole new dynamic in the One Piece universe with this new power.
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.
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.
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.