Well, a Goku - One Piece crossover fanfiction might involve Goku getting lost in the One Piece universe. He could end up on a random island and befriend Luffy and his crew. Goku would probably be interested in the different Devil Fruits and might even try to use his powers to help them find the One Piece. His fighting skills would also be a great asset during battles against the likes of the Yonko or the World Government's forces.
Definitely the potential for epic battles. Picture Goku going up against the Four Emperors all at once. His speed and strength could turn the tides in a battle that the One Piece world has never seen before. Also, his friendship with Luffy could lead to some really heartwarming moments. They both share a love for adventure and fighting strong opponents. And Goku's ability to sense energy could be a game - changer in a world where most rely on observation Haki. It could open up new ways of detecting threats and finding hidden things in the One Piece universe.
One possible scenario could be Goku Black using his power to take over a part of the One Piece world. He might try to find the One Piece itself, thinking it could grant him even more power. His dark nature would likely put him at odds with the Straw Hat crew, leading to epic battles.
Goku would probably be very friendly with Luffy. They both love to fight strong opponents and have a sense of adventure. Goku might be a bit confused by some of the wacky things in the One Piece world like the Devil Fruits at first, but he'd quickly adapt. He'd also likely be impressed by the determination of the other characters like Zoro and Sanji.
A Naruto - One Piece fanfiction could explore the differences in their power systems. While Naruto has chakra-based abilities, the pirates in One Piece rely on Haki, Devil Fruits, and physical strength. It would be fascinating to see how Naruto would interact with the various characters in the One Piece universe. For example, how would he get along with Zoro, the swordsman? Or how would he deal with the wily Nami when it comes to treasure? There are so many possibilities for an exciting and unique story.
One interesting crossover could be Goku arriving in the One Piece world and teaming up with Luffy to fight a powerful enemy. Goku's incredible strength and combat skills combined with Luffy's Devil Fruit powers would make for an epic battle. Another could be Goku teaching the Straw Hats some of his martial arts techniques, which would enhance their fighting abilities in their own adventures.
Goku Black could be a powerful new threat. His ability to instantaneously move between spaces using his powers would be like a more advanced form of the Shanks' ability to appear suddenly. He could easily target the weak points of the various islands in the One Piece world.
For Luffy, I think a Pikachu would be a great fit. Just like Luffy, Pikachu is full of energy and has a really positive attitude. They both are also very brave and not afraid to take on opponents much stronger than themselves. For Zoro, a Scizor would be suitable. Scizor is a strong and battle - hardened Pokemon, much like Zoro with his swordsmanship skills. Nami could be paired with a Meowth. Meowth loves shiny things, just like Nami loves money and treasure.
Sakura would fit in as a strong fighter. Her physical strength and combat training from being a ninja would make her a formidable opponent in the pirate - filled world of One Piece. She could hold her own in battles against pirates or other threats.
One interesting concept is a crossover with other popular anime like Naruto. Imagine Luffy and Naruto teaming up, it would be a clash of two very different power systems and personalities. Another concept could be a crossover with a superhero universe. For example, Luffy meeting Superman. Their powers are so different, it would make for an exciting story.