Firstly, it should start with mutual respect. Given Luffy's nature, Fem Luffy would respect Hancock's position as the empress of Amazon Lily. Hancock, on the other hand, would respect Fem Luffy's strength and determination. For example, in a battle situation, they could see each other's abilities up close.
Another plot could involve a common enemy. Maybe a new powerful pirate group attacks both Amazon Lily and Fem Luffy's friends. Hancock and Fem Luffy have to work together to defend. During the battle, Hancock realizes Fem Luffy's strength and leadership skills, and they gradually fall in love as they fight side by side. After the battle, they start to explore their relationship more deeply.
They could be united by a common enemy that is not related to the main plot of One Piece. Maybe an ancient power or a group of pirates from a far - off land that threatens both the Amazon Lily and the Straw Hats. Working together against this new threat would make their relationship deeper as they rely on each other's strengths. For instance, Hancock's knowledge of the sea around her island and Luffy's fearless combat style would be a great combination.
Well, a lot of these fanfictions play with the dynamic of Hancock's strong personality and the fem reader's unique traits. For example, some might have the reader be a new pirate in Hancock's territory, and Hancock becomes intrigued by this different person. You could also search for ones that are tagged with specific themes like 'friendship' or 'romance' to find the kind of story you're interested in.
First, know the characters well. Hancock is a proud and powerful character in One Piece. For the fem reader, you can create a distinct personality. Make sure the relationship between them develops in a believable way. Maybe start with an interesting encounter, like the fem reader getting lost on Hancock's island.
One great fanfic could be 'Luffy and Hancock's Grand Adventure'. It might explore their relationship in a new and exciting way, perhaps with them going on a quest together that deepens their bond. Another could be 'Luffy's Surprise Encounter with Hancock' which could focus on a chance meeting that changes their dynamic.
One idea could be that Hancock's feelings for Luffy lead her to secretly help the Straw Hats in a dangerous situation. For example, when they are facing a powerful enemy on an island, Hancock uses her influence and powers to divert the enemy's attention.
Often, 'One Piece Kid x Fem Luffy fanfiction' has a focus on character development. Fem Luffy might develop a more assertive side when dealing with Kid, and Kid could become more empathetic. It also can bring out different aspects of their Devil Fruit powers in relation to each other, like how they could combine their abilities in battles or use them to overcome obstacles in their journey together.
First, establish their different backgrounds clearly. Hancock has her history as a Shichibukai and her experiences with slavery. The Fem OC could come from a different part of the One Piece world with her own set of beliefs. For their relationship to develop, there should be a gradual build - up of trust. For example, the Fem OC could save Hancock from a difficult situation, and Hancock in turn could offer her protection. They should also have moments of conflict, like when they disagree on how to handle a certain situation, and then resolve it in a way that brings them closer.
One idea could be that the Fem OC is a new pirate who arrives on Hancock's island. At first, Hancock is wary but as they interact, they find common ground in their strength and independence. Maybe the OC has a special ability that Hancock respects. Another plot could involve a threat to Hancock's kingdom, and the Fem OC helps her defend it, leading to a deepening of their relationship.