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.
Another concept could be that the Nine - Tailed Fox from Naruto somehow gets loose in the One Piece world and its chakra affects Devil Fruits, causing strange mutations. Luffy and his crew then have to team up with Naruto and his friends to find a way to stop the chaos.
One idea could be a story where Pikachu ends up on the Grand Line in the One Piece world. He teams up with Luffy and his crew, using his electric attacks to help them in battles against the Marines or other pirates. Another could be a fanfic where the Pokemon trainers are like pirates, traveling from island to island in the One Piece world to catch different Pokemon species that are native to those islands.
One idea could be a character from the One Piece world somehow obtaining the Rinnegan. This character could then use the Rinnegan's powers, like the ability to see chakra, in the context of the One Piece world. For example, they might be able to detect the 'life energy' of Devil Fruits or sense the hidden strength of characters like Luffy's Gear forms. It would create a whole new dynamic in the One Piece universe with this new power.
One idea could be that the male OC is a former slave who was freed by Whitebeard. He has a deep sense of loyalty towards Whitebeard and is always ready to fight for his newfound family. His unique ability could be related to some form of water - based combat, which is rare in the One Piece world.
One idea could be that the Straw Hat Pirates end up in the Marvel Universe and team up with the Avengers. Maybe Luffy's rubber powers could be useful in a battle against a common enemy like Thanos. For example, he could stretch to reach far - off enemies or use his elasticity to absorb powerful blows.
The character of Butch could inspire a tattoo too. You might consider getting his boxing gloves as a tattoo. It could be a detailed design on your bicep, showing the worn - out look of the gloves. This would represent his fighting spirit and his role in the movie's complex story.
Maybe there could be a situation where the Devil Fruits in One Piece are related to the Hollow powers in Bleach. Some of the characters in One Piece start showing Hollow - like traits due to a strange phenomenon. Zoro, for instance, might start having an inner struggle similar to how Ichigo battles his inner Hollow. And the Bleach characters have to help the One Piece crew figure out what's going on and how to control these new powers.
You could also explore an OC Admiral who is a bit of an outlier in the Navy. Maybe they have a more lenient view on pirates and believe in reform rather than just punishment. This would create a lot of internal conflict within the Navy and interesting interactions with the Straw Hat crew when they cross paths.
Some fan novels might explore the backstories of the existing characters in more depth. For instance, we could learn more about Zoro's training with his old master in more detail. Maybe there are secrets about his training that were never revealed in the original series. This would give fans a better understanding of his character and his unwavering determination.