The OC could be a young inventor who comes to Fish - Man Island to study its unique technology. There, they meet Shirahoshi. The OC creates a device that helps Shirahoshi communicate more easily with the surface - dwellers. In return, Shirahoshi shows the OC the hidden secrets of the Fish - Man Island, and they form a strong friendship that transcends the differences between humans and fish - men.
One common theme is adventure. Since One Piece is all about adventure, many fanfictions with Shirahoshi OCs involve her going on new quests or exploring uncharted areas in the One Piece world.
Sure. You can try searching on fanfiction websites like Archive of Our Own. There are often many great works about Shirahoshi's original character interpretations there.
First, focus on the relationship between Shirahoshi and the OC. Decide if it's a friendship, a mentorship, or something more romantic. For example, if it's a friendship, they could have adventures together exploring the underwater world. Second, introduce a conflict. It could be something like an evil pirate group threatening Fish - Man Island or a misunderstanding within the fish - man society about the OC. Third, let the characters grow. Shirahoshi might become more confident with the OC's support, and the OC could learn about the importance of protecting the ocean and its inhabitants.
One idea could be a fanfiction where a captain from One Piece accidentally sails into the Naruto world. He then decides to recruit some of the powerful ninja as part of his crew. This could lead to some really interesting dynamics as the ninja have to adjust to life on a ship and the rules of the One Piece world.
One idea is to make the OC a long - lost heir to the royal family. They could have been hidden away for protection and now return to claim their rightful place. This brings in elements of mystery and adventure.
One idea could be that your pet Doflamingo has a unique ability that helps the Straw Hat crew in a strange way. For example, it can sense danger from far away, like when the crew is about to be attacked by the Marines. It might use its flamboyant appearance to distract the enemies while the crew escapes.
Another possibility is that the One Piece world has its own version of the Juubi. The poneglyphs could hold secrets about this powerful entity. A character like Robin, with her ability to read the poneglyphs, discovers information about the 'One Piece Juubi'. Then the story could follow how the different pirate crews react to this new threat or opportunity, and how they might interact with Naruto characters if they were to cross over.
Another concept is an immortal villain. In the Naruto - One Piece crossover fanfiction, this villain could be a powerful figure who has been around for ages, manipulating events in both worlds. They might be using the chaos of the ninja wars in Naruto and the power struggles among pirates in One Piece to further their own dark goals. They could be invincible to normal attacks due to their immortality, forcing the heroes from both universes to find a special way to defeat them.
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.
Maybe in the fanfiction, the One Piece characters could gain some of the abilities from Naruto. For example, Luffy could learn how to use Rasengan. And the Hokage characters would have to adapt to the strange rules of the Grand Line in One Piece, like the different sea currents and islands with their own unique ecosystems.