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.
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.
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.
There is also 'Castiel's Choice'. This one is interesting as it focuses on Castiel having to make some tough decisions that directly impact his relationship with Dean. It's full of emotional moments and really makes you think about the depth of their connection within the Supernatural universe.
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.
I really liked 'In the Shadows with Meg and Castiel'. It's a more introspective piece that explores their individual struggles and how they come to rely on each other. The descriptions in the fanfiction are really vivid, making you feel like you're right there in the Supernatural universe with them.
There's also a story where Castiel kidnaps a fan in a desperate attempt to save the world. It's a complex plot that involves time travel and alternate universes. You can find it on FanFiction.net. The story really delves into Castiel's character and his motives behind the kidnapping.
I remember reading a fanfic called 'Castiel's Wounds'. In this story, Castiel is severely injured during a battle with demons. The fic focuses on how Sam and Dean try to help him recover, and the relationship dynamics between the three are really well - written. It shows Castiel's vulnerability in a very believable way. There's also 'A World of Hurt' which explores Castiel's hurt in a post - apocalyptic setting, adding an extra layer of tension and drama.
I really like the fanfic 'Castiel's Endless Burden'. It shows Castiel constantly feeling the pressure of being different from other angels due to his relationship with the Winchesters. It's a story full of angst and self - discovery for Castiel, and the way it's written makes you feel his pain.
One great Castiel - dead fanfiction is 'The Aftermath of an Angel's Fall'. It really delves into how the other characters in Supernatural deal with Castiel's death. It shows Dean's inner turmoil and Sam's struggle to come to terms with it.