One great Naruto, Admiral, and One Piece fanfic is 'Cross - Universe Adventures'. It combines the unique elements of each universe in an exciting way. The characters from Naruto, like Naruto himself, interact with the admirals from One Piece in really interesting battle scenarios. Another good one is 'All - World Convergence' which delves deep into how the different power systems of these three universes would interact and balance out.
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.
The exploration of new storylines is what makes these fanfics interesting. With immortality involved, the story doesn't have to be limited by the normal lifespan of a character. So, it can span across different eras of both Naruto and One Piece universes. Imagine an immortal Naruto character joining Luffy in his search for the One Piece hundreds of years after the events of Naruto. It could open up all kinds of uncharted territory for the story, like how the world has changed in that time, new villains that have emerged, and how the character's immortality affects their relationships with others.
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.
A good Naruto, Admiral, and One Piece fanfic stands out when it blends the different power systems well. For example, if it can make Naruto's ninjutsu work in harmony with the devil fruits and admiral powers in One Piece. Also, when it has great character crossovers. Picture Luffy and Naruto having a friendly race or competition. And unique plotlines that involve all three universes are a plus.
The Rinnegan could be integrated as a mysterious power that emerges in the One Piece world. Maybe it's tied to a long - lost treasure or a forgotten civilization. A character could discover it and start to unlock its abilities, gradually changing the power balance in the Grand Line.
One idea could be that the Straw Hats end up in the Naruto world and have to team up with the ninja to fight a common enemy. For example, they might face a powerful demon that can only be defeated with both Devil Fruit powers and ninja jutsu. Another idea is that some of the Naruto characters get Devil Fruits. Like Naruto getting a Logia type fruit that allows him to transform into a powerful energy form, similar to his Nine - Tails chakra mode but with new abilities.
In a 'Pirate Captain Naruto x One Piece fanfic', Naruto's leadership as a pirate captain would be a key element. He would have to deal with the challenges of the sea, like storms and sea kings. His relationship with his crew would be different from what we see in Naruto. In the One Piece world, there are different power systems and politics. Naruto would have to adapt to that. His ninjutsu could be seen as a strange and powerful art by the people in One Piece. There could be a lot of potential for creative storylines, such as Naruto trying to find a way back to his own world or trying to change the power balance in the One Piece world with his ninja abilities.
Well, a really cool crossover in a Pirate Naruto x One Piece fanfic could be the sharing of abilities. For example, if Zoro was able to learn some chakra control from Naruto, he might be able to enhance his sword attacks with an extra energy source. Also, the ninjas from Naruto's world could teach the One Piece characters some stealth techniques, which would be very useful for the Straw Hats when they are sneaking onto an enemy ship.
One Piece's serial time was longer than Naruto's. One Piece started in 1997 and Naruto started in 2007. Therefore, One Piece's serial time was about 10 years longer than Naruto's.