One common theme is love. Since Boa Hancock has a crush on Luffy, many fanfics explore how this love could develop. Another theme is adventure. Luffy is always on an adventure, and fanfics often involve Boa joining him in his escapades.
A big theme is also about breaking barriers. Hancock is a proud and strong woman from Amazon Lily, and Luffy is a carefree pirate. Their relationship in fanfics often shows how they break cultural and social barriers between them. For example, Hancock's tribe has certain traditions and views about men, but with Luffy, she defies some of those. Also, friendship is a theme as Luffy values friendship highly and Hancock becomes part of his circle of friends in a special way.
One popular 'Boa Hancock x Luffy' fanfic is 'The Pirate's Heart'. It explores their relationship in an interesting way, with great character development for both. Another one could be 'Love on the Grand Line' which has some really sweet moments between them.
I'm not sure specifically which 'luffy x boa lemon fanfic' is the best. But you can try searching on some popular fanfic websites like FanFiction.net or Archive of Our Own. Maybe look for ones with high ratings and positive reviews.
Often, the theme of adventure might be intertwined. Given that Luffy is all about exploring and Boa has her own story in the One Piece world. Their relationship could be set against the backdrop of new adventures they embark on together. Another theme could be the discovery of emotions. Luffy is not always in touch with his romantic feelings, and Boa has her own complex emotions, so fanfics may focus on how they come to terms with their feelings for each other.
One popular 'boa and luffy fanfic' is 'The Pirate's Heart'. It focuses on their unique relationship and the adventures they could have together. Another one might be 'Boa's Obsession' which delves into how Boa Hancock's feelings for Luffy could develop in different scenarios.
In the One Piece world, Luffy is on this grand adventure and Hancock is a powerful figure. Their relationship has this allure of the unexpected. Fans are drawn to the idea of these two very different characters coming together. Fanfic writers can play with their personalities and create all sorts of interesting scenarios. For example, Hancock's initial haughty attitude towards most people but her special soft spot for Luffy makes for great storytelling. It's also about the idea of love in a world full of pirates and battles, which is quite appealing.
There is a fanfic called 'Love on the Grand Line'. In this story, Hancock uses her powers to protect Luffy during a dangerous battle. This event makes Luffy see Hancock in a new light. It also delves into how the Straw Hat crew reacts to Hancock's obvious affection for their captain, which adds an interesting dynamic to the story.
One popular 'boa hancock luffy fanfic' is 'The Pirate's Heart'. It explores the unique relationship between Luffy and Hancock in a really interesting way. It has great character development and some exciting adventure scenes.
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.