Common themes include Castiel's quest for knowledge. He's always been curious about humanity and the world around him, and fanfics expand on that. The theme of sacrifice is also there. He's sacrificed a lot for the Winchesters and for the greater good in the show, and fanfics play with that concept. Another one is his connection to Heaven and how it affects his actions. His ties to the other angels and his role in the angelic hierarchy are often explored, as well as his attempts to break free from or reform that hierarchy.
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 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 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.
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.
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.
One common theme is loyalty. In many of these fanfictions, Dean and Castiel show unwavering loyalty to each other, whether it's facing the forces of evil in the Supernatural universe or dealing with personal issues. Another theme is sacrifice. They are often willing to sacrifice themselves for the other's well - being.
One common theme is Castiel's struggle with his identity. His angelic nature vs his growing humanity can often be a trigger for panic attacks. For example, in many fics, he might panic when he realizes he's feeling emotions that are more human - like, such as fear or love in a way that angels aren't supposed to.