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 about Luffy having an important conversation. Maybe it's about his dreams, his crew, or some kind of revelation he has. Without reading it, it's hard to say exactly.
I'm not sure if there is such a specific fanfiction. There are a vast number of fanfictions out there, but this particular concept might be quite rare or not in line with the general tone of the 'One Piece' series. Fanfiction often explores different relationships and scenarios, but this one seems rather out - of - the - ordinary considering the nature of the characters in the original work.
A third theme could be facing greater challenges. In these fanfictions, the challenges are so big that Luffy can't just rely on his normal, laid - back self. He has to be more serious to overcome them.
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.
Obviously, Luffy and Hancock are the main characters. Luffy, the captain of the Straw Hat Pirates, known for his carefree and determined nature. And Hancock, the beautiful and powerful Pirate Empress. Their personalities play a big role in the story, especially when dealing with the situation of pregnancy.
The wedding would probably be a huge and chaotic affair. There would be pirates from all over coming to celebrate. Luffy's friends like the Straw Hat crew would be there, of course. There would be lots of food and drink, especially meat for Luffy. The venue might be on an island they've visited before, like Sabaody Archipelago or even on the Thousand Sunny.
One great 'luffy gets serious' fanfiction is 'The Serious Luffy Adventure'. In it, Luffy faces a powerful new enemy that forces him to drop his usual carefree attitude and strategize. It shows a different side of Luffy's character.
In some fanfiction scenarios, if Nami were to side with an enemy, even if it was for a seemingly good reason like trying to protect the crew in a way that Luffy doesn't understand. Luffy has a strong sense of loyalty to his crew and seeing Nami on the 'opposing side' might enrage him. He might feel betrayed and his anger would be a natural reaction.