Well, often the main characters are still the Straw Hat crew. But in Black Thunder fanfiction, there might be some original characters added. For example, there could be a character named Thunderbolt Jack who has the ability to control black thunder and is a sort of anti - hero. He might start as an enemy to the Straw Hats but later become an ally.
In some Black Thunder One Piece fanfiction, there could be a story where the Straw Hats discover a hidden island during a thunderstorm. This island is filled with ancient artifacts and a mystery related to the Black Thunder. As they explore, they have to deal with various traps and enemies that are also after the secrets of the island.
The Black Pearl could be a treasure in the One Piece world that somehow ends up in the Naruto world. This could cause chaos as the shinobi are not used to such a valuable and strange object. Pirates from One Piece might then travel to the Naruto world to retrieve it, leading to all sorts of interesting interactions between the two groups.
One possible fanfiction could be about a crossover where characters from Naruto and One Piece encounter a mysterious Black Pearl. Maybe the pirates from One Piece are searching for this powerful pearl which has some sort of chakra - like energy. And the ninjas from Naruto are also interested because it could enhance their jutsu. They end up in a battle for the pearl, but then realize they can work together to understand its true power.
Often, you'll see the main character as a new pirate captain who takes over the Black Pearl. Maybe it's a character with a troubled past, like being exiled from their own pirate crew. Another main character could be a navigator who has a special bond with the ship.
In the world of One Piece, there's no indication that the characters themselves watch fanfiction. Fanfiction is a creation of the fans in the real world, and the in - universe characters are too busy with their own adventures, like Luffy chasing after the One Piece, Zoro training to be the world's greatest swordsman, and Nami navigating the seas. They don't have access to or knowledge of the fan - made stories about them.
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.
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.
In One Piece Black Pearl fanfiction, a great concept could be the Black Pearl being a symbol of hope. The crew that sails it might be a group of outcasts trying to prove themselves in the pirate world. They could use the ship's mysterious powers, like being able to disappear in the fog, to outwit their enemies and get closer to the One Piece.