One common theme is family. Since it's about Luffy's brother OC, family bonds are often explored. The OC may have a different upbringing but still shares a deep connection with Luffy. Another theme is loyalty. The OC usually ends up being loyal to Luffy and his crew, just like the other Straw Hats.
In a One Piece fanfic, an OC (Original Character) as Luffy's rival could be really interesting. This OC might have a unique Devil Fruit ability that directly opposes Luffy's powers. For example, if Luffy has the Gum - Gum Fruit which gives him stretchy abilities, the rival could have a hardening or shrinking ability. This would create intense battles between them as they fight for different ideals on the Grand Line.
One possible fanfic could be about Luffy's brother Ace meeting Boa Hancock. Maybe Ace could be on a mission near Amazon Lily and they cross paths. He could be initially curious about her powers and her reputation as a beauty. Another idea is a fanfic where Hancock falls for Luffy's brother because he shows some of the same fearless and kind traits as Luffy, but in a different way. It could explore how their relationship develops in the complex world of One Piece.
One common theme is adventure. Since it's set in the One Piece world, the OC and Boa often go on exciting adventures together. Another theme is love or at least a strong attraction. Their relationship, whether it's a slow - burn or an instant connection, is usually a central part of the story.
Well, I've heard of 'A Meeting with Boa'. In this fanfic, the OC's backstory is intertwined with Boa Hancock's. It starts with an unexpected encounter on an island. The author does a good job of creating tension and chemistry between the two characters. It's definitely worth checking out if you like this kind of fanfic.
One great 'One Piece Luffy brother OC fanfic' is 'The New Brother in the Straw Hats'. It adds an interesting original character as Luffy's brother who has unique abilities like being able to control water, which is a new concept in the One Piece world. This OC's journey with the Straw Hats brings a lot of fresh and exciting adventures.
Another way is to check out One Piece fan communities on platforms like Reddit or Discord. There are often threads where fans share and recommend good fanfics. You can post a request asking specifically for fanfics about Luffy's brother and Boa Hancock, and I'm sure fellow fans will be happy to help you out.
One interesting plot idea could be the OC brother having a unique power that contrasts with Luffy's. For example, while Luffy has his rubber powers, the brother could have the ability to control metal. This would lead to interesting interactions within the Straw Hat crew, like helping Usopp upgrade his weapons. Another idea is that the OC brother has a different view on the One Piece treasure. He might be more interested in the historical aspect of it rather than the wealth or power it represents. This could cause some internal debates within the crew as they journey towards the Grand Line.
The OC brother could fit in as a strategist. Luffy is more of an action - first kind of leader, so having a brother who can think several steps ahead in battles and planning voyages would balance the crew's decision - making. For example, in dealing with the Navy's traps, the brother could analyze the situation and come up with a smart escape plan.
In a One Piece fanfic, an OC can be a worthy rival to Luffy if they have a strong and unique personality. Luffy is carefree, loyal, and always positive. The OC could be the opposite, like being cold, calculating, and self - centered. Their clashing personalities would not only create interesting interactions but also drive the rivalry forward as they compete in the pirate world.