The theme of self - sacrifice also comes up a lot. Castiel's burden often leads him to sacrifice his own well - being, whether it's his physical form, his grace, or his standing among the angels, all in the name of protecting the Winchesters and the world from various threats.
A common theme is the struggle between his angelic duties and his loyalty to the Winchesters. Castiel often has to choose between what his angel brethren expect of him and what he believes is right in helping Sam and Dean.
One common theme is the idea of unconditional support. Castiel is often depicted as being there for Dean no matter what, through his darkest moments. For instance, when Dean is haunted by his past actions or when he's feeling extremely guilty.
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 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 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.