Love is also a frequent theme. Sometimes it's a romantic love between the OC and Castiel, and other times it's a more platonic, but still very strong, love. In these fics, their relationship evolves over time as they share experiences and learn more about each other in the complex and dangerous world of Supernatural.
One common theme is redemption. The OC often has a past they need to make up for, and Castiel helps them in that journey. For example, in many fics, the OC might have accidentally caused harm in the past and is now seeking a way to right their wrongs with Castiel's guidance.
Another theme is friendship. Since Castiel is such an important character in Supernatural, the OC usually forms a deep bond with him. They become friends and support each other through all the dangerous situations they face in the story, like fighting off powerful demons or dealing with angel politics.
One common theme is redemption. Since Castiel has made some mistakes in the show, a lot of fanfiction focuses on him trying to make up for those. Another theme is his relationship with the other main characters, like Dean and Sam. His loyalty and how it is tested is often explored.
One common theme is Castiel's struggle with his angelic nature and his growing humanity. He often has to choose between following angelic orders and doing what he believes is right. Another theme is his loyalty to the Winchesters, especially Dean. His relationship with Dean is a central focus in many fanfics, whether it's a deep friendship or something more. Also, themes of redemption are common, as Castiel has made many mistakes in the series and fanfics often explore his path to redemption.
One common theme is the clash of different worlds. Castiel, from the dark and often gritty world of Supernatural, finds himself in the bright and magical world of MLP. This contrast often leads to themes of adaptation and understanding.
The theme of unrequited love can be common. Maybe Castiel has feelings for someone who doesn't return them. Guilt is also a big one. He might feel guilty for actions he took that had negative consequences. And the idea of being an outcast. Since he's different from both angels and humans in many ways, he often feels like an outcast, which brings about a lot of sadness in fanfictions about him.
Another theme is friendship. His relationships with the other characters from Supernatural and Ghostfacers are often explored. How he bonds with Sam, Dean, or the Ghostfacers crew is a big part of many fanfictions. In some, he might form a really strong friendship with a new character introduced in the fanfic world.
Isolation is a very common theme. Since Castiel is abandoned, he often finds himself alone, cut off from his former celestial family. This isolation makes him question his identity and purpose.
Protection is also a big theme. Castiel might use his 'nest' to protect something or someone he cares about. It could be a hiding place for a character in danger or a place where he guards important items or secrets related to the 'Supernatural' world.
The theme of discovery is also prevalent. The OC often discovers the existence and significance of Castiel's wings, which can be a journey of self - discovery for both characters. It might also lead to the discovery of new aspects of the Supernatural world they inhabit, like hidden places or powers that are related to the wings.
Another theme is the symbolism of his wings. Some fanfictions explore how his wings represent his angelic nature and his connection to Heaven. They might also delve into how the state of his wings reflects his emotional and spiritual state, like broken wings when he's feeling lost or damaged.
One common theme is the exploration of their deep emotional connection. Since they have a strong bond in the show, fanfictions often delve into how that bond could turn into a romantic relationship. For example, stories might focus on how they deal with their feelings in a world where hunters usually don't have time for love.