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.
Well, Sasuke is a complex character. In a One Piece fanfiction, his backstory of seeking revenge could be paralleled with some characters in One Piece. For example, he might form an alliance with characters who also have a vendetta against a powerful enemy. His ninja skills could be adapted into unique fighting styles in the pirate - filled world of One Piece. Maybe he'd be a bounty hunter at first, and then get involved with the Straw Hats through some unexpected turn of events.
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.
Sasuke could be a foreign exchange student of sorts. His Sharingan might be seen as a very rare and powerful magical ability. He would likely be very good at Defense Against the Dark Arts due to his combat skills.
One plotline could be that Sasuke is from a hidden ninja - like island in the One Piece world. He is sent out on a mission to find a Devil Fruit that has the power to unlock the secrets of his clan's past. Along the way, he crosses paths with the Straw Hat crew and initially clashes with them due to his cold and brooding nature.
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.
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.
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.
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.
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.
Sasuke would be a very powerful addition to the RWBY world's power structure. His Sharingan gives him enhanced perception, which would be extremely useful against the Grimm. He could easily analyze their attack patterns and weaknesses. Also, his ninjutsu skills like the Chidori and Fire Style: Great Fireball Jutsu are very powerful. In terms of the Huntsmen and Huntresses ranking, he would probably be considered an S - class level threat or fighter due to his abilities.
They could fit in as a new type of pirate. The Rinnegan's powers would give them an edge in combat and exploration. For instance, they could use the Preta Path to absorb attacks from devil fruit users, like Logia types. This would make them a feared opponent on the high seas.