They could meet at a local library. Beckett is there looking for books on detective work (even as a kid she has an interest in it), and Castle is there for the latest adventure novels. They accidentally reach for the same book, and this starts a conversation. They find out they both have a love for stories and mysteries. As they grow up, this shared interest might be what brings them closer when they meet again in their adult lives.
Well, it could be that they meet at a summer camp. Beckett is the shy girl who loves to read mystery books in the corner, and Castle is the adventurous boy always getting into little pranks. They might team up to solve a small mystery at the camp, like who stole the counselor's whistle. This initial meeting could plant the seeds for their future relationship as crime - solving partners.
Maybe they meet at a school event. Castle could be showing off his creative writing skills in a school play he wrote, and Beckett, being a smart and inquisitive kid, is really interested in his story. They start talking about how to make the story better, and from there, a friendship forms. This encounter early in life might be what draws them together years later in the world of crime - solving.
As kids, Beckett might be more vulnerable and less guarded. She's not yet been through the trauma that shapes her as an adult, so she could be more open to new friendships and experiences. Castle, on the other hand, would be more boyish and carefree, not as suave as he is in adulthood. He'd be more likely to make silly mistakes and be less smooth with his words.
The plot could be centered around a misunderstanding. Beckett might think Castle is interested in someone else, and this causes some tension between them. But as they work through the case, they start to realize their true feelings for each other. The first kiss could come when Castle finally clears up the misunderstanding, and they share a tender moment.
The fanfic could start with Castle and Beckett already having a baby. They are adjusting to their new life as parents. Beckett might struggle a bit with the lack of sleep and trying to balance work and motherhood. Castle, on the other hand, is loving every moment of being a father. He writes a new book based on their experiences of having a baby. Meanwhile, they also have to deal with some family drama, like an over - eager relative who wants to be too involved in the baby's life. But through it all, their love for each other and for their baby only grows stronger.
Perhaps in the fanfic, Beckett bends over in a metaphorical sense, like she's going out of her way to support Castle. It could involve her putting aside her pride or her own goals to help him with a case or a personal matter. For example, she might be willing to take on a dangerous undercover assignment that she normally wouldn't just to protect Castle or further his investigation.
One possible plot could be that Castle sees Beckett with Josh and it makes him realize his true feelings for her. He might then start to compete for her attention in a humorous and endearing way, like writing a new novel inspired by his jealousy and love for her.
In this fanfiction, the baby boy could be the center of a family drama. Maybe Castle's ex - wife shows up, claiming she has some rights to the baby. This causes a rift between Castle and Beckett as they deal with the legal and emotional turmoil. However, in the end, they come together stronger than ever, with their baby boy being the glue that holds their family together.
There could be a moral dilemma in the story. What if the kidnapper demands something unethical in exchange for the child? Castle and Beckett would have to decide how far they are willing to go to save their child. This could test their relationship and their values. But in the end, they would find a way to outsmart the kidnapper and get their child back safely.
One possibility could be that Castle and Beckett face the challenges of parenting while still dealing with their detective work. Maybe they have to take the baby to the precinct at times, causing some comical situations among the colleagues.
Maybe Castle realizes he can't live without Beckett and confesses his love at the last minute, which stops the wedding. For example, he could show up with some big revelation about their relationship that makes Beckett question going through with the wedding.
Maybe Castle's pregnancy is part of a prophecy in the fanfiction world. Beckett, being the skeptic, has to come to terms with this strange turn of events. The two of them then embark on a journey to figure out what the prophecy means for their future and the future of their unborn child. This would involve a lot of mystery and exploration.