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.
A good fanfiction also needs a strong plot. You could start with a conflict that involves all three aspects. Say, a powerful enemy that is a combination of the threats in Naruto and One Piece is threatening the New World. Develop the relationships between the characters from both series. Let them learn from each other and grow. For instance, Zoro could teach Sasuke some new sword techniques, while Naruto could inspire Luffy with his never - give - up attitude.
If Naruto were in the One Piece world, he could form interesting alliances. He might team up with Luffy and the Straw Hats. Naruto's ability to make friends easily would fit right in with the Straw Hat crew's spirit. His Rasengan could be a very powerful attack against some of the strong foes in the One Piece world. Also, his experience with dealing with different types of enemies in his own world would give him an edge in strategizing in the new and strange world of One Piece.
In a fanfiction where Naruto is a fishman in One Piece, Naruto would face new challenges. For example, dealing with the discrimination that some fishmen face in the One Piece world. But he'd also have new opportunities. He could use his fishman physiology to swim faster and deeper, discover hidden treasures, and maybe even find a way to connect the ninja world with the pirate world. His Nine - Tailed Fox chakra might also have some interesting interactions with the strange powers in One Piece.
It might reimagine the One Piece world by introducing elements from other universes. For example, new types of Devil Fruits inspired by powers from other worlds. Maybe a Devil Fruit that gives the power of telekinesis like some characters in a sci - fi universe have.
Naruto's ninjutsu skills could be very powerful in the One Piece world. His Rasengan, for instance, is a concentrated ball of chakra. In the One Piece world, it could be as strong as some of the more powerful Devil Fruit attacks. His ability to run up walls and trees using chakra control could also be useful in the more diverse terrains of the One Piece world, like running up the side of a ship or a rocky island.
I think 'One Piece War of the Best Fanfiction' could center around a war that has the best elements from the One Piece world. It could involve the clash of ideologies among the pirates. For instance, some pirates might be fighting for freedom in a different way compared to Luffy's crew. And there could be a complex plot where different alliances are formed and broken during this war, with the fate of the One Piece world at stake.
The One Piece system could change the relationships between characters. Heroes and villains who previously had no connection might be brought together because of the new elements introduced by the system. For example, the X - Men might have to team up with a group of characters inspired by One Piece pirates to face a common threat that the One Piece system has brought into the Marvel world. This could lead to new storylines and character developments as they learn to work with different personalities and power sets.
Madara's powers would be both a boon and a bane in the One Piece fanfiction world. His ability to summon the Susanoo could be a fearsome sight on the Grand Line. It could be as intimidating as some of the giant sea kings or even the ancient weapons. On the other hand, his powers might not be fully understood by the characters in One Piece. For instance, the concept of chakra is different from Haki and Devil Fruits. But if written well, his presence could add a whole new layer of excitement and mystery to the story. Maybe he could form alliances with some of the Yonko or even oppose them, depending on the plot of the fanfiction.