Perhaps their daughter has a special talent that gets her involved in a dangerous situation. She could be a prodigy in computer hacking, and while trying to help her parents on a case, she uncovers a secret that puts her life in danger. Castle and Beckett then have to race against time to protect her while also solving the case.
They might disappear because of a time - traveling device gone wrong. So they end up in a different era, say the 1920s. Castle could be a struggling writer and Beckett a detective in a very different kind of police force. They have to adapt to the new time while also trying to figure out how to get back to their own time. Or perhaps they disappear due to a magical curse. In this case, they have to solve the mystery of the curse to reappear in their world.
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.
One possibility is that they are trapped in a virtual reality world. Beckett is trying to figure out how to get them out while also keeping his daughter calm. They might have to solve puzzles or defeat digital threats. This could explore how their family bond helps them in this high - tech, yet entrapping situation.
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.
There are many fanfictions that explore Castle and Beckett starting a family. Some might focus on their journey through pregnancy, with Beckett dealing with the physical and emotional changes. Others could be about how they adjust to life with a new baby, like the sleepless nights and the joys of seeing their child grow.
Jealousy could also play a part. Castle is often surrounded by beautiful and adoring fans as a famous writer. Beckett might start to feel insecure, especially if she sees some of these fans getting a bit too close to Castle. On the flip side, Beckett's dedication to her work and her male partners in the police force could make Castle jealous, ultimately leading to their break - up.
Perhaps the man is a new partner Beckett has been assigned to work with on a case. Castle sees them together and his insecurities get the best of him. He starts imagining all sorts of things, like Beckett replacing him in her life. However, in the end, it turns out that Beckett has no romantic feelings for the other man and Castle realizes he overreacted.