One possible plot could be that the pregnancy is a result of some sort of magical or otherworldly event. Castiel might feel a strong sense of responsibility to protect Meg and the unborn child. He could use his angelic powers to keep them safe from various threats in the Supernatural universe, like demons or other malevolent forces.
I really like 'A Meeting of Opposites'. In this fanfiction, the author explores the idea of Castiel and Meg having to work together on a mission. It's a great read because it plays with their personalities. Castiel's naivete and Meg's cunning are both highlighted and as they interact, it creates a lot of tension and some really cool plot twists.
Character development is crucial. In fanfiction, we get to see different sides of Meg and Castiel. Maybe we see a more vulnerable side of Castiel or a less evil - more complex side of Meg. And the plot usually revolves around some sort of conflict, either between them or against a common foe.
In fanfiction, if Sam was kidnapped, it could lead to a very intense story. Dean would surely go on a rampage to find him. Maybe Sam was kidnapped by a powerful demon who wants to use him for some dark ritual. It could also be a new, unknown enemy who has a vendetta against the Winchesters.
The fanfiction might show Meg dealing with the physical and emotional changes of pregnancy while still being her tough self. She might have to fight off those who would try to harm her because of the baby. Castiel, on the other hand, would be by her side, using his powers when necessary. It could be a story of their growth as individuals and as a couple.
Another popular theme is the power struggle between them. Meg is a powerful demon and Castiel is an angel with his own set of powers. Fanfictions sometimes focus on how they use their powers against each other, or in some cases, how they learn to combine their powers for a common goal, which makes for some really exciting action - packed stories.
Dean is almost always going to be in these fanfictions when Sam is gagged. Their relationship is central to Supernatural. Rowena could also be a character. She might use her magic to either gag Sam for her own purposes or to help him if she has a change of heart. And Gabriel. He could turn up and use his trickster abilities to either put Sam in the gagged situation as a joke or to get him out of a more serious one.
One great fanfiction is 'Sam and Rowena's Unlikely Alliance'. In this story, they team up to face a powerful new threat that the Winchesters alone can't handle. It has a lot of great character development for both Sam and Rowena, showing their complex relationship in a new light. The author does a wonderful job of capturing their voices, making it feel like an extension of the show.
One great fanfiction could be where Dean and Sam have to face a unique Halloween - themed monster. Maybe a witch who uses Halloween traditions to cast powerful spells. They would have to use their wits and hunting skills to stop her before she causes chaos in a small town.
There's this fanfiction called 'Sam and Jack: Unlikely Allies'. It starts with them being at odds with each other, but as the story progresses, they start to understand and respect one another more. The plot is well - paced, and the author does a great job of writing their dialogue in a way that feels natural to their characters.