Castle might become more determined to solve the mystery behind her death. This would drive him to dig deeper into the case, no matter how dangerous it is. His character would shift from being the somewhat carefree writer to a more focused and vengeful investigator.
He could become more withdrawn. Losing Beckett like that would make him hesitant to let people in again. He'd be more guarded, fearing another loss.
His view on life would change drastically. He'd no longer take things for granted. He might start to question his own purpose. Before, Beckett was a big part of his life and motivation. Now that she's gone, he has to find a new reason to keep going. He could also develop a darker side, being more cynical about the world and the people in it, until he eventually finds a way to heal and grow from this tragic experience.
Castle might become obsessed with finding the truth about what happened. His writer's mind would start formulating theories and he would stop at nothing to get justice for Beckett, which could put him in dangerous situations.
Castle could initially deny it's her. He'd look for any sign that it's not Beckett, checking for details that are off. As he realizes it is her, he'd break down, crying uncontrollably. His world would shatter because she was such an important part of his life, and he'd be filled with regret for any unsaid words or things they didn't do together.
He would likely feel extreme shock. Just the sight of her body could send his mind into a tailspin. He might also be in denial at first, not wanting to believe that it's really her.
One challenge could be the state of the body. If it has been damaged or decomposed, it would be difficult to identify. Another might be the lack of proper identification tools or information at the scene. Maybe there are no fingerprints available or dental records are missing.
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.
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.
In a fanfic, their first kiss might occur during a more emotional moment. For example, Beckett could be sharing a deep, personal story from her past. Castle listens intently, and when she finishes, he just leans in. It's a kiss that's both a comfort and a new beginning, a way to show that he's there for her not just as a partner in crime - solving, but also as someone who cares deeply about her on an emotional level.
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.
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.
Maybe he saw Beckett spending a lot of time with another man without knowing it was for a case. Castle is a bit of a jealous type, and he might overreact to normal interactions.