Love and friendship are also very common themes. Whether it's a platonic love or a more romantic love, it's often explored. In some fanfics, they have to hide their feelings from others, which adds an element of secrecy. And then there's the theme of sacrifice. They are both willing to sacrifice themselves for the greater good and for each other.
One common theme is loyalty. They are always there for each other in most fanfics, no matter what the situation is. Another theme is the struggle between the human and the supernatural world as Castiel is an angel and Dean is a human hunter.
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.
One common theme is sacrifice. They often sacrifice for each other in these fanfics, like Castiel giving up his grace or Dean putting himself in danger to save Castiel. Another is the exploration of their unspoken feelings. Since they don't always express their emotions openly in the show, fanfics love to dig into that.
One common theme is the struggle with identity. Castiel, being an angel, has to come to terms with his love for a human, which goes against the nature of angels in some ways. Another theme is sacrifice. Often in these fics, Castiel is willing to sacrifice a great deal for Dean because of his love.
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 their relationship. It could be a romantic relationship, a deep friendship or a more complex bond that defies easy categorization. Another theme is the struggle between good and evil, with both Dean and Castiel often fighting on the side of good but having to face their own inner demons as well.
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.
One common theme is loyalty. Castiel is often depicted as extremely loyal to Dean, and this loyalty is tested in various ways in the fanfics. Another theme is sacrifice. They are willing to sacrifice for each other. For example, in many stories, Castiel might sacrifice his angelic powers for Dean.