The main characters would be Beckett and Castle, as the title implies. Beckett is likely to be the one taking some sort of action in relation to Castle.
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.
The main characters are Castle and Beckett, obviously. Castle is likely to be the one who intervenes in Beckett's wedding. Beckett is the one whose wedding is being stopped.
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.
Esposito and Ryan are very likely to be involved. They are Castle's friends and colleagues, and they would probably be there to support the couple. They might offer their own brand of humor and help out in practical ways, like throwing a baby shower for Beckett and Castle. Also, Martha, Castle's mother, would surely be a central figure. She could offer her wisdom on pregnancy and child - rearing, and add some comical moments with her unique personality.
There could be. The Castle fandom is quite creative, and a pregnancy plot for Beckett is a popular trope in fanfiction. Some fanfics might focus on the romantic aspect of it, with Castle being over - the - moon and taking extra care of Beckett. Others might explore the more practical and challenging sides, such as Beckett still trying to solve cases while dealing with pregnancy symptoms.
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.
He could react with anger. After all, if he has feelings for Beckett, seeing her with someone else like Vaughn might make him furious. He might lash out at both Beckett and Vaughn without really understanding the situation.
It could be a vengeful criminal that they put behind bars. This person has escaped and wants to get back at them, so they engineer their disappearance. Maybe they have a whole elaborate plan to torture them in some remote location.
A coworker who has a crush on Beckett might be a main character. They could be the one who stays by Beckett's side the most during his recovery, and this could lead to a romantic sub - plot.
One possibility in a Castle and Beckett jealous fanfic could be a new character entering the scene. For example, a famous detective from another precinct comes to help on a case. This detective might show a romantic interest in Beckett, which makes Castle jealous. He starts to act out, being overly protective and a bit more sarcastic than usual. Beckett, on the other hand, might not notice Castle's jealousy at first, thinking he's just being his normal self. But as time goes on, she realizes his feelings and it could lead to a big confrontation between them, which ultimately strengthens their relationship.