The popularity comes from the unique relationship between Luffy and Vivi in the original 'One Piece' series. Their friendship is very special, and fanfics allow fans to explore it further.
There's also 'Luffy and Vivi: Reunion'. This fanfic focuses on what happens when Vivi reunites with the Straw Hat crew after a long time. It beautifully captures the emotions and the unbreakable friendship between Luffy and Vivi, with some really exciting action scenes in between.
The OC can be developed by having them learn from Vivi's strong sense of loyalty and duty to her kingdom. For example, if the OC was a bit of a carefree spirit before, seeing Vivi's dedication could make them more responsible. They could also develop their fighting skills by training with Vivi, who is no stranger to combat situations in the One Piece world.
One possible storyline could be that the OC is a wandering adventurer who saves Princess Vivi from a new threat in Alabasta. They then team up to discover the source of this threat which turns out to be an old enemy of the kingdom who has returned with a new power. As they journey together, they develop a deep friendship and respect for each other.
Friendship is a very common theme. Their unwavering loyalty to each other is often explored. For example, in many fanfics, Zoro will always be there to protect Luffy and vice versa.
In a One Piece fanfic, an OC (Original Character) as Luffy's rival could be really interesting. This OC might have a unique Devil Fruit ability that directly opposes Luffy's powers. For example, if Luffy has the Gum - Gum Fruit which gives him stretchy abilities, the rival could have a hardening or shrinking ability. This would create intense battles between them as they fight for different ideals on the Grand Line.
In One Piece, the relationship between Luffy and Nami is complex. There are many instances where they show great care for each other. For example, Nami is often worried about Luffy when he gets into dangerous battles. Luffy, on the other hand, will do anything to protect Nami. While it may not be an out - and - out love story like in a traditional romance, there is definitely a strong and unique relationship that has elements of love in the form of deep friendship and loyalty.
One common theme could be friendship evolving into something deeper. Since Ace and Luffy have a strong bond in the original 'One Piece' story, in these fanfics, it might be explored as a more romantic or intimate relationship. Another theme could be sacrifice, as both characters are known for their selfless natures in the anime.
There's also 'Vivi and Luffy: A Tale of Friendship and More'. This fanfic explores their relationship not just as friends but delves into the potential of a deeper romantic connection. It might include scenes where Vivi's actions towards Luffy are more than just friendly gestures, like how she always looks out for him in a special way.
FanFiction.net is also a good place. There are numerous One Piece fanfictions on this site. Look for the One Piece section and then search specifically for 'vivi loves luffy' within that. However, the quality may vary, so you might need to skim through a few to find the really good ones.