A Sharingan user could be a new type of pirate. They could use the Sharingan to read the Log Pose and navigate the Grand Line more accurately. For example, by seeing the magnetic fields or something similar with their Sharingan enhanced vision.
Perhaps a Sharingan user could be a marine in the One Piece world. They would be a very effective interrogator, using the Sharingan's genjutsu to extract information from pirates. They could also use it in combat against powerful pirates, like being able to anticipate their attacks better.
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.
He might fit in as a sort of super - strong outsider. Vegeta could be on a quest for his own kind of 'One Piece', which could be something related to power or a way to return to his own universe. His Saiyan abilities would make him a force to be reckoned with. For example, his ability to transform into Super Saiyan forms would be like a new level of power that no one in the One Piece world has seen before. He might start out as an antagonist but could end up teaming up with the Straw Hats if they had a common enemy.
Sakura could fit in as a new crew member. Her strong will and determination, which she showed in Naruto, would be valuable traits on the Straw Hat ship. She could also bring a different fighting style to the crew, adding variety to their combat capabilities.
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.
Naruto could fit in by using his physical abilities. His superhuman strength, speed, and endurance, which he developed through his ninja training, would make him a formidable opponent in the One Piece world. He could easily take on weaker pirates or even some of the lower - level Marines.
Sasuke's character could potentially bring a new level of intensity to the One Piece world. His Sharingan abilities, like seeing through illusions and predicting movements, might make him a formidable opponent. He could be a lone wolf type of character in the Straw Hat crew, initially aloof but gradually warming up to the others, much like Zoro in some ways.
She'd probably fit in quite well. Her strong will and determination are similar to many characters in One Piece. She could use her ninja skills to make a name for herself in the pirate world. Maybe she'd become a sort of legend, like a new type of pirate with ninja abilities.
One way it could fit is by having a character with a quirk that allows them to mimic the Sharingan's abilities. For example, they might be able to copy other heroes' or villains' quirks in a similar way to how the Sharingan can copy jutsu in Naruto. This would add a whole new level of strategy and intrigue to the My Hero Academia world.
She could fit into the power structure by forming alliances. Since the One Piece world is full of various factions, Lucy could side with the Straw Hat Pirates. Her powers would be useful in their adventures. She might also encounter other groups like the Revolutionary Army. Her celestial spirit magic could be used for espionage or gathering information for them. And as she travels more in the One Piece world, she could learn about new ways to enhance her magic using the unique resources there.
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.