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.
Robin is crucial in the One Piece novel. Her knowledge of the Poneglyphs makes her a key figure in the search for the One Piece. She can decipher the ancient texts that hold secrets about the Void Century and the true history of the world. This knowledge not only aids the Straw Hat crew in their adventure but also makes her a target for the World Government.
I like the storylines where Robin forms deep friendships in the fanfictions. In one, she befriends a group of scholars on an island. They exchange knowledge and she helps them protect their research from pirates. It's a great display of her character values. There are also storylines that explore her relationship with the other Straw Hats in new ways, like how she teaches Usopp about ancient plants.
Another angle could be a romantic relationship between them. It would be interesting to see how their relationship develops, considering their different backgrounds. For example, Robin's past with the Straw Hats and Hancock's role as a Shichibukai. Their relationship could bring out new sides of their characters that we haven't seen before in the original story.
Another option could be a new character introduced specifically for the fanfiction. This character could be a powerful pirate or a mysterious figure from Hancock's past. He might have a connection to Hancock's family or her position as a Shichibukai. His presence would add an element of mystery and new dynamics to the story, especially in relation to the pregnancy.
In a Mihawk x Boa fanfiction, the plot could revolve around a mission. Let's say the World Government tasks Mihawk with keeping an eye on Boa because they suspect she might be planning something against them. Mihawk, being the stoic character he is, accepts the mission. Boa, on the other hand, quickly realizes she is being watched. At first, she tries to get rid of Mihawk, but he proves to be a difficult adversary. As time passes, they start to understand each other's motives. Boa's reasons for her actions might be related to protecting her island and people, and Mihawk might start to respect her for that. This could lead to a change in their relationship from one of animosity to mutual understanding and maybe even love.
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.
Robin and Luffy have some great interactions in fanfiction. Luffy's silly antics can often make Robin laugh, which is a rare sight given her usually calm demeanor. For example, in some fanfics, Luffy might accidentally stumble upon Robin's reading corner and start asking her all sorts of curious questions about the books, leading to an endearing exchange between them.