One common theme is the relationship between Castiel and Dean, with Misha's character often playing a significant role in their dynamic. Another is Castiel's journey to understand human emotions better, which can be explored in many different ways in fanfiction.
The theme of redemption is quite common too. Castiel has made many mistakes throughout the series, and fanfiction often delves into his attempts to make up for those. And of course, there are the crossovers. Some fanfictions combine the Supernatural universe with other shows or movies, and Castiel and Misha's character are integrated into these new and exciting storylines. This can bring a whole new perspective to the character and the story.
A lot of fanfictions focus on Castiel's angelic powers and how they interact with the human world. Misha's portrayal of Castiel often inspires stories about his character's internal struggles, like his loyalty to Heaven versus his newfound friendships on Earth. There are also many stories that explore the concept of Castiel falling in love, which can be a very interesting theme to read about.
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.
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.
Romantic relationships between them are also a popular theme. Writers might explore how a relationship between an angel (Castiel) and a hunter (Dean) would work, dealing with the differences in their natures, their loyalty to their respective worlds, and how they overcome obstacles to be together.
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.
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.