Another theme could be the relationship between Dean and Castiel in this different time period. They might have to hide their true natures more, especially Castiel as an angel. Their bond could be explored in the context of the 50's secrecy and social norms. Maybe they have to pretend to be something they're not just to fit in while still trying to carry out their missions.
One common theme is the contrast between the 50's era and the supernatural world. The 50's had a certain aesthetic and set of values, and when combined with the Supernatural elements, it creates an interesting dynamic. For example, Dean's more modern - day sensibilities might clash with the more conservative 50's society while he's on a hunt with Castiel.
One common theme is the struggle between good and evil within Dean. Castiel is often portrayed as the guiding light trying to pull Dean back from the dark side. Another theme is sacrifice. There are often situations where either Dean or Castiel has to sacrifice something important for the other's sake.
One great 'Dark Dean Winchester Castiel' fanfiction is 'The Darker Path'. It delves deep into the complex relationship between Dean and Castiel in a darker, more angsty setting. The story is filled with emotional turmoil and challenges for both characters as they face a new and menacing threat.
One common theme is friendship. Their bond, whether it's a platonic friendship or something more, is often explored. Writers like to show how they support each other through tough times.
Some fanfictions focus on how their marriage affects the rest of the Supernatural universe. For example, how their relationship impacts Sam or other characters. There are also themes related to their different natures - Dean as a human and Castiel as an angel - and how they navigate their married life with these differences, such as Castiel trying to understand human emotions better for the sake of their marriage.
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 the bond between Dean and Sam. When Dean is kidnapped, Sam's determination to save him is always a central part. It shows the depth of their brotherly love. Another theme is the exploration of Dean's vulnerability. Usually, he's seen as strong, but when kidnapped, his weaknesses are exposed. For example, in some fics, he has to face his fears of being alone or tortured.
A common theme is friendship. Often, the stories focus on how the reader becomes friends with Dean and Castiel. Another theme is adventure. Since they are hunters in the Supernatural world, they go on many adventures together with the reader. For instance, they might go on a quest to find a powerful artifact.
One common theme is the exploration of forbidden love. Since Castiel is an angel and Dean is a human, their relationship often goes against the norms of both their worlds. Another theme is sacrifice. They are often willing to sacrifice a great deal for each other.
Love and acceptance are also very common themes. Dean may struggle at first with his feelings for fem Castiel, but eventually comes to accept and love her for who she is. There's also the theme of power. How does fem Castiel's power change or stay the same in relation to Dean? And how do they use their combined powers in different situations, like during hunts or facing off against supernatural enemies.
One common theme is redemption. After Castiel hurts Dean, he often tries to redeem himself, which shows the depth of his love and friendship for Dean.