I'm not sure specifically about 'lemon' fanfiction as it often refers to inappropriate or adult - themed content. However, there are many great Luffy x Makino fanfictions that focus on their relationship in a more appropriate way. You can check out popular fanfiction websites like FanFiction.net or Archive of Our Own and search for them there.
You can anticipate stories that delve into their backstories and how they intersect. For example, Makino knew Luffy since he was a child, so fanfictions might explore how that early connection influenced their relationship as Luffy grew older. Some might also show Makino's role in Luffy's life beyond just being someone from his hometown, perhaps as a source of emotional support during his tough times at sea.
I would recommend 'Ace, Luffy and Sabo: Reunion of Souls'. This fanfiction is full of heart - warming moments. It shows how their past experiences shape their present interactions and how they support each other in the face of difficulties.
I really like the fanfic 'The Bond of Luffy and Hancock'. In this one, their relationship is explored through various challenges they face together. It shows how Hancock's tough exterior softens when it comes to Luffy. There are also some interesting side characters that add depth to the story. Then there's 'Luffy's Impact on Hancock'. This fanfic focuses on how Luffy's presence changes Hancock's world view and how she grows as a person because of him.
I really like 'Luffy and Law's Shared Destiny'. It shows how their fates could be intertwined in a more romantic way. The story has some really sweet moments between them, like when they share their dreams under the starlit sky during a break in their journey. It also adds new elements to the One Piece world they exist in.
Rebecca could potentially be in Luffy's harem. After Luffy helped her in Dressrosa, she might develop feelings for him. She is a strong - willed and kind - hearted character. Another could be Shirahoshi. Luffy has a special connection with her as he saved her from being captured. Her gentle and innocent nature would contrast with some of the other more assertive members of the harem.
Their relationship could develop through mutual respect. Luffy respects Whitebeard as a powerful and respected pirate, and Whitebeard respects Luffy's determination and potential. For example, they could start with a simple exchange of stories about their adventures at sea.
Another theme is rivalry. Zoro and Sanji have a friendly rivalry which can be elaborated on in fanfictions. For instance, they might compete in a friendly tournament on the ship to see who is stronger or more useful in a certain situation.
I'd recommend 'The Secret Mission of Makino and Shanks'. This fanfic has exciting action scenes and a touching emotional core. The author does a great job of capturing the essence of both characters.