Well, perhaps in the fanfiction, Luffy and Hancock end up on an adventure together and during that time, Hancock realizes she's with child. Luffy might not fully understand at first what it means, but his friends on the Straw Hat crew help him. The story could then revolve around how they prepare for the baby's arrival, with Hancock teaching Luffy about family and Luffy sharing his dreams of a free world with the future baby.
In this fanfiction plot, Hancock and Luffy could be on a mission to find a powerful treasure that is said to protect their future family. Along the way, Hancock discovers she's going to have a baby. The Straw Hat crew, along with the Kuja tribe, come together to create a safe haven for Hancock during her pregnancy. Luffy has to learn to be more responsible as he awaits the birth of his child. He starts to think about how he can make the world a better place not just for his friends, but also for his new family.
One possible plot could be that Hancock discovers she's pregnant after a strange encounter on an island. Luffy, being Luffy, is initially confused but then excited. They could face challenges from the World Government who fear the power of their child. Hancock would use her Kuja abilities to protect their unborn baby while Luffy battles those who oppose them.
One possible plot idea could be that Hancock discovers she is pregnant with Luffy's baby. This could cause a stir in Amazon Lily as they deal with the idea of their empress having a child with an outsider. Meanwhile, Luffy might be oblivious at first, still on his adventures, until the news reaches him and he has to figure out how to be a father while still pursuing his dreams of becoming Pirate King.
One possible plot could be that Hancock's island is in danger from a new and powerful enemy. Luffy, being the heroic type, comes to her aid. Along the way, they discover a new type of Devil Fruit power that they have to figure out how to deal with. Luffy's simple - minded yet determined approach and Hancock's pride and strength would make for an interesting dynamic as they face this new threat together.
In fanfiction, if Luffy and Hancock got married, it could be a really fun and unexpected story. Maybe Luffy would still be more interested in adventures and food, and Hancock would have to find a way to deal with his carefree nature. Their relationship might bring new dynamics to the world of One Piece. For example, the Kuja tribe could become more involved in Luffy's grand plans.
In a fanfiction, their relationship might develop when Luffy gets injured and Hancock has to take care of him. This would be a new situation for Hancock as she is usually the one being protected. Luffy, in his injured state, would still be his positive self. Hancock would see his vulnerability and also his unwavering spirit. This could lead to a deeper emotional connection between them, with Hancock realizing she wants to be by his side not just because of her love for him, but also because she admires his character.
One idea could be that Hancock's feelings for Luffy lead her to secretly help the Straw Hats in a dangerous situation. For example, when they are facing a powerful enemy on an island, Hancock uses her influence and powers to divert the enemy's attention.
Well, one possible plot could be that after many adventures together, Luffy realizes his deep feelings for Hancock. They might have a big wedding on Amazon Lily with all of Luffy's crewmates attending. Hancock would be overjoyed to finally marry the man she loves, and Luffy would be happy in his own simple way, excited for the new chapter in their relationship and the feasting that would come with the celebration.
If they had a baby, Hancock would become more protective not just of Luffy but also of their child. Luffy would have a new sense of responsibility. He might not be as reckless as before, at least when it comes to the safety of his family. Their relationship would deepen as they share the joys and concerns of raising a child in the dangerous world of One Piece.
Yes, there are many fanfictions exploring a romantic relationship between Luffy and Hancock. Some writers like to imagine how their different personalities would interact in a romantic context. Hancock is smitten with Luffy in the original story, and fanfic writers often expand on that, creating scenarios where Luffy gradually realizes his feelings for her too.
One possible plot could be that Hancock discovers she's pregnant and decides to keep it a secret from most people at first. She then goes on a journey to find a safe place away from the chaos of the pirate world to raise her child. Along the way, she meets some old friends and enemies, and has to use her wits and strength to protect herself and her unborn baby.
One possible plot could be that Luffy and Hancock get married and start a family. Their family could then embark on adventures together, with their children inheriting some of their unique abilities. For example, their son might have Luffy's rubber powers and Hancock's beauty - related abilities, and they could use these to protect their island or go on treasure hunts.