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.
In many sad Castiel fanfictions, the theme of broken relationships is present. His relationships with the Winchesters or other angels can break down, leaving him in a state of sadness. The theme of self - doubt also pops up. Castiel may doubt his own abilities and decisions, which leads to his melancholy. And powerlessness. When he can't save the day or protect those he loves, he feels sad and that's a common theme too.
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.
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.
Betrayal is also a theme. Sometimes he is betrayed by those he trusts, which causes great sadness. For example, when his fellow angels turn against him. And there's the theme of unrequited love. His feelings for certain characters may not be reciprocated, adding to his sorrow. This is often explored in 'castiel sad fanfic' to show the depth of his emotional turmoil.
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.