Well, Chopper in a really comical and out - of - the - box fanfic could transform into a human girl (through some wacky magic or something) and give Luffy a kiss just for fun. Usopp's sister (if we assume he has one in the fanfic) who has heard all about Luffy's adventures and is a big admirer could be the one kissing him. And then there's always the chance that it could be a random girl from an island they visit. Maybe she's a baker's daughter and Luffy saved her bakery from being burned down by pirates, so she gives him a kiss as a thank you.
One possibility could be Nami. Since they've been through so much together on the Straw Hat crew, an emotional moment might lead to a kiss. Another could be Robin. She has a special bond with Luffy, and in the context of a fanfiction, a kiss could be used to show their deep connection.
Rayleigh would be a great candidate. He's an experienced pirate and has already trained Luffy for a while. He has a lot of knowledge about the world of pirates and could give Luffy a talk about the deeper secrets of the Grand Line or how to deal with the powerful enemies that lie ahead.
It could be his own mother. She might realize that he has become too dangerous and, in a moment of heart - breaking decision, find a way to destroy him. Maybe she discovers a weakness and uses it against him, driven by her love for humanity and the knowledge of the destruction he could cause.
It could be Vegeta. Vegeta might kick Goku out of a training session because he feels Goku is showing off or not taking the training seriously enough. Vegeta has always been competitive with Goku and this could be his way of asserting his dominance.
One possible storyline could be a moment of extreme emotion. Maybe they are in a dangerous situation, like facing a powerful enemy. In the heat of the moment, as a sign of comfort and connection, they share a kiss. It could be a one - time thing that then makes them both realize new feelings for each other.
Another option might be Robin. She has a calm and intelligent demeanor. Robin and Luffy share a mutual respect. Her knowledge and Luffy's leadership could make for an interesting dynamic in a marriage within the fanfiction world. Also, they both have a common goal of finding the One Piece in a way.
Another possibility could be a new character introduced in the fanfic. This new character might be using Naruto to gain something, like power or influence in the ninja world, and when they get what they want, they cheat on him. It would add an element of mystery and betrayal to the story.
In this fanfiction, Luffy might be used as a bargaining chip by his captors. Maybe they'll try to exchange him for something valuable, like a powerful Devil Fruit or a large amount of treasure. His crew would then surely launch a rescue mission, facing many challenges along the way.
In some 'luffy first kissed fanfiction', it might be part of a plot device to show his growth. Maybe he kisses someone as a sign of a new understanding or connection. For example, if he kisses a character from a different island they visit, it could symbolize a new alliance or friendship. However, different authors will have different takes on this concept, so there's no one - size - fits - all answer.
It could be the Marines. They've been after Luffy for a long time, so it's possible they finally manage to capture him in this fanfiction.