A plotline could also revolve around how the baby affects their relationship with their friends and colleagues. Some might be over - enthusiastic about the baby, while others might feel left out. Castle and Beckett would have to navigate these social dynamics while also taking care of their new family member. Plus, there could be a sub - plot where Beckett struggles to balance her career as a detective with being a new mother.
One possible plotline could be that the baby has some sort of special ability or is in some way connected to a mystery they have to solve. Maybe the baby's birth coincides with a strange series of events in the city, and Castle and Beckett have to use their skills to protect the baby and figure out what's going on.
It could also be a fanfic that takes place in an alternate universe. In this universe, the events of 4x23 didn't happen as they did in the show. Maybe Beckett didn't become a detective in the same way, and Castle meets her in a completely different situation. They then start to solve mysteries together in this new setting.
A Castle and Beckett baby fanfic could be about their journey into parenthood. It might explore how they adjust to having a baby in their lives. For example, Castle could be super excited and start writing new stories inspired by his new family experience, while Beckett might be a bit more cautious and protective.
One possible plotline could be that Castle, in his over - the - top way, proposes something like a wild undercover mission where they have to pretend to be a couple in a very improper situation to solve a case. Beckett is initially shocked but then realizes it might be the only way to catch the criminal.
Good Castle Beckett baby fanfic often focuses on the emotional aspect. It shows how their love deepens with the addition of a baby. For example, their shared joy and the way they support each other through the challenges of parenting.
In a fanfic where they have a baby, their relationship would shift. They would now be partners not only in crime - solving but also in parenting. Castle would likely become more domesticated, helping with diaper changes and midnight feedings. Beckett would have to learn to let Castle in more on the parenting front, and they would communicate more about family - related matters. This new aspect of their relationship would add a different layer of complexity and closeness to their bond.
Maybe they stumble upon a government conspiracy. The government wants to keep something hidden and they are seen as a threat, so they are made to disappear. It could also be related to a scientific experiment gone wrong. They are in the wrong place at the wrong time and get caught in an energy field that transports them to an unknown location, causing them to disappear from their normal world.
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 they are trying to protect their relationship from outside interference. For example, if there are people in Castle's publishing world or Beckett's police department who don't approve of their relationship, they could pretend to be separated to avoid any negative consequences. This would allow them to still be together in private while keeping up the appearance of being single in public. Also, it could be a way to test their relationship. They might want to see how they would fare on their own and if their feelings for each other are strong enough to withstand a period of 'separation'.
One possible reason could be that they are on a secret mission. Maybe they are trying to protect a key witness or important evidence from a powerful and dangerous criminal organization. In the world of fanfic, this could lead to them having to disappear to keep their whereabouts unknown.
Another plot twist could be that the child wasn't really kidnapped but was hidden away by a family member for some strange reason. And as Castle and Beckett dig deeper, they find themselves caught in a web of family secrets and lies that has more to do with protecting the child from a greater danger that they are not yet aware of.