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.
Maybe even one of his crewmates. Zoro, for example, could give Luffy a talk about determination and never giving up. He might share his own experiences and how he overcame difficulties, which could inspire Luffy and also give him some new perspectives on their journey together.
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.
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.
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.
Lupin could be a likely candidate. Given his experience and his role as a sort of mentor figure to Harry, he could give Harry the talk. It might involve Lupin sharing his own experiences with being a werewolf and how to deal with secrets and prejudices in the wizarding world.
Axel could be the one to give Xion 'the talk'. He's been through a lot and has a good understanding of the world they live in. He could share his experiences with her, especially about loyalty and friendship, as he values his relationships with Roxas and Xion deeply.
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.
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.
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.
She could be a former pirate captain herself. Maybe she was one of the most feared pirates in her time, and her skills and reputation were passed down to Luffy in some way, even if he didn't know it was from his mother.