A really interesting plot could be that they disappear because they are pulled into a virtual reality world created by a mad genius. In this world, everything seems normal at first, but they start to notice glitches. They realize that if they don't find a way out, they will be trapped forever. Along the way, they encounter versions of people from their real lives, but with different personalities. This makes it difficult for them to trust anyone as they search for the exit of this virtual prison.
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.
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.
A cute plot could be that their baby is born during a big case. Castle is away when Beckett goes into labor, and he has to race back. When he finally arrives at the hospital, he is just in time to hold their new - born, and they both realize how their lives are about to change in the most wonderful way.
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.
A third plot point could be Beckett turning around in her view of Castle's family. At first, she might be a bit hesitant or reserved about getting involved with his wacky family. But then something happens, like she spends a holiday with them and sees how much they care for her and Castle, and she has a change of heart and fully embraces them as part of her life too.
One idea could be that Beckett's friends have to use their unique skills to outwit the kidnappers. For example, if Beckett has a tech - savvy friend, they could hack into the kidnappers' communication system to find out their location. Another idea is that Beckett discovers a hidden strength within themselves during the hostage situation and manages to escape on their own, only to then lead the authorities back to rescue the other hostages.
One idea could be a story about a young adventurer who discovers a hidden passage in Bast Castle. They then embark on a journey to find out what secrets the passage holds, perhaps leading to a long - lost treasure.
One idea could be a mystery within the Arcadia Castle. Maybe a valuable artifact has gone missing and the main character has to search through the castle's many hidden passages and rooms to find it. Another could be a love story set against the backdrop of the castle's grandeur, with two characters from different social classes falling in love while attending a ball at the castle.
There's also 'Beyond the Case'. This fanfic takes their story to new heights by adding some interesting plot twists outside of the normal case - solving scenarios. It shows how their relationship might develop in different, more challenging situations. It's a great read for fans who want something a bit more out of the ordinary.
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.
A common plot element in such a fanfic could be misunderstandings. For instance, if Castle is seen having lunch with an old female friend, Beckett might misinterpret it as a romantic encounter. She could become distant, which in turn makes Castle confused and then jealous of the attention Beckett gives to her work instead of him. This simple misunderstanding can spiral into a series of events where both are trying to make the other jealous without actually communicating their true feelings.