One common theme is friendship. Sanji and Luffy have a strong bond in the original series, and in fanfics, this bond is often explored more deeply, sometimes in a way that blurs the line into a more affectionate relationship. Another theme could be adventure. They might go on new and unique adventures together that are not part of the original story.
Growth is also a theme. They both grow as characters in the original story, and in fanfiction, it can be shown in unique ways. For example, Sanji might learn something from Luffy about leadership, and Luffy might learn about the importance of good food from Sanji, and these learnings can be part of a more complex character - development arc within the fanfiction.
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.
Another theme is the mutual respect they develop for each other's skills. Sanji's cooking and Law's medical and strategic abilities are things they come to admire in one another. For example, in many fanfics, Law is initially dismissive of Sanji's cooking but then realizes its value. It's like in 'The Taste of Understanding' fanfic where Law first scoffs at Sanji's food but later it becomes a crucial part of their bond.
One common theme is mutual understanding. Since they both have rather complex pasts, fanfics often explore how they come to understand each other on a deeper level. For example, Sanji might understand Robin's need for solitude at times because of his own experiences of being alone in the kitchen.
Another theme is self - discovery. Through their relationship, both Sanji and Pudding often learn more about themselves. For example, Sanji may discover new depths to his emotions, and Pudding might find a new sense of purpose outside of her family's expectations.
One common theme is the idea of unrequited love at first. Often, Sanji is shown pining after Nami while she is initially oblivious or just sees him as a friend. Another theme is the growth of their relationship through shared adventures. As they face various challenges together on the ship and during battles, their bond deepens.
One common theme is their friendship. They are all part of the Straw Hat crew and fanfictions often explore how their different personalities interact and support each other. For example, how Zoro's stoicism and loyalty play off against Luffy's carefree leadership and Sanji's chivalry.
One common theme is brotherly love. Since they are brothers in the One Piece universe, fanfics often explore the depth of their love and loyalty for each other. Another theme is growth. They both grow as characters, and fanfics may show how their relationship helps in that growth. For example, Luffy might become stronger because of Ace's influence.
In many 'robin x luffy fanfic', the theme of protection is present. Luffy is always ready to protect his crewmates, including Robin. And Robin, in her own way, also tries to protect the crew and Luffy. Their relationship in the fanfics also often reflects the theme of loyalty. They are loyal to each other and to the crew. This loyalty is what makes their relationship so endearing in the fanfics and is a key element that the writers like to explore.
One common theme is friendship. Luffy and Chopper have a very special bond in the original series, and fanfics often explore and expand on that. They might write about how they support each other during tough times on their adventures.