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.
In terms of survival, Naruto characters are used to living in a world where they have to fend for themselves. In the Highschool of the Dead world, they could use their Sharingan to quickly assess threats and their ninja skills to find food and shelter. They might also teach the other high school students some basic self - defense techniques.
Maybe by creating false visual illusions. For example, using some basic ninja techniques like the Transformation Jutsu to disguise as someone important to the Sharingan user. This could make the Sharingan user misjudge the situation and fall into the trap.
The Sharingan in Naruto time travel fanfiction can be a game - changer. It might give Naruto the power to see the future of the past. Let's say he travels back to a time when a great war is about to start. With the Sharingan, he can foresee the strategies of the enemy leaders and counter them before they are even implemented. It also adds an interesting layer to the story as Naruto has to learn to control this new - found power related to the Sharingan in a different time setting, and it can lead to some really exciting plot twists as he uses it in unexpected ways.
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.
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.
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.
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.
The Sharingan can fit by enhancing a character's perception. In fanfiction, it could make a wizard like Harry more aware of his surroundings during duels. He could see the flow of magic more clearly, similar to how in the Naruto universe, Sharingan users can see chakra.
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.
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.