Luffy's carefree and adventurous spirit might make Boa both frustrated and fascinated. He might just barge into her life without much thought, like he does with most things. Boa, on the other hand, would be used to a more formal and respectful approach. So it would create an interesting clash of personalities.
In fanfiction, Luffy's character in Gear 5 might develop a greater sense of responsibility. Since Gear 5 is so powerful, he realizes he has to be more careful about using it. For example, he might avoid using it in areas where there are a lot of innocent bystanders.
In some harem fanfictions, Luffy may develop more emotional intelligence. For example, he has to understand the feelings of the different girls in his harem. He might become more perceptive about relationships compared to his rather straightforward self in the original series.
His leadership style could change significantly. In the original, he leads with his heart and by example, but in a dark fanfiction, he could be more of a dictator - like leader within his crew, making decisions without much input from others and demanding strict obedience. His goals might also shift from just finding the One Piece and having a grand adventure to seeking power for more self - centered reasons, like revenge or dominance.
Female Luffy may have a different body shape. Instead of the muscular build of the original Luffy, she could be more slender but still strong. Her hair might be styled differently too, maybe longer or in a different color. Also, her clothing choices could be more feminine while still maintaining the pirate - like style.
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.
The theme of acceptance is also present. Hancock has a rather difficult past and her feelings for Luffy make her question a lot of things about herself. Fanfictions will sometimes focus on how Luffy's simple and accepting nature helps Hancock come to terms with her own identity and insecurities, and how she in turn becomes more integrated into Luffy's world.
😋I recommend the following two novels to you: One Piece: The Revolving Fruit and One Piece World's Cultivation Journey. The main character of One Piece: The Twirling Fruit was Luffy's subordinate. In the later stages of the game, he was a boss, but he was not invincible. The main character in One Piece World was also Luffy's partner. However, the main character was gifted and broke through the established ignorance and rebellion. He was a gentle version of One Piece. I hope you like this fairy's recommendation. Muah ~😗
In some fanfictions, their interactions are centered around knowledge - sharing. Since Robin is highly intelligent and Luffy is always eager to learn new things, Robin might explain to Luffy about the history of different islands they visit. For example, she could tell him about the origin of the strange creatures or the significance of the local architecture.
Well, in such fanfiction, Luffy's character often becomes more aware of his emotions. Usually, he is so focused on adventure and being the Pirate King, but when he falls in love, he starts to think about someone else's feelings. For example, he might be more hesitant to go on extremely dangerous adventures without considering how his loved one would feel.
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.