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.
If it involves sex - related talk, then it's not appropriate as it goes against the family - friendly nature of the original Luffy character and the One Piece series in general.
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.
In Luffy fanfiction, Shanks might talk about his hopes for Luffy's future as a pirate. He could mention how he saw the potential in Luffy when he gave him the straw hat.
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.
It could be Dumbledore. He's always been like a mentor to Harry, so it would make sense for him to be the one to give an important talk, whether it's about Harry's destiny or how to handle the challenges in the wizarding world.
This fanfiction could center around Harry getting a talk on some moral or ethical issues. Given the complex world of magic, there are many gray areas. So perhaps a character is having a deep conversation with Harry about the right and wrong use of magic, or how to handle the power he has been given. It could also be related to his identity as a wizard, and how to balance his Muggle - born and wizard heritage.