The theme of sacrifice often shows up. Whether it's Castiel sacrificing for Dean or vice versa, it adds an emotional depth to the story. Also, the exploration of the human - angel relationship in a new light with the gender change. It can bring out different aspects of loyalty, trust, and the blurring of boundaries between their two very different existences.
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 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.
Another theme is the struggle between their different natures. Castiel being an angel and Dean being a human hunter. This creates a lot of internal and external conflict in the stories, like in 'The Clash of Worlds' fanfiction where they have to overcome their differences to be together.
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 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 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.
Another theme is the idea of identity. How does Castiel's female form change her perception of herself and how does Dean view her differently? It can lead to stories that are centered around self - discovery and acceptance. In some fanfictions, Castiel struggles with being in a female body and Dean helps her come to terms with it, which is a really interesting take on their relationship.
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.
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.
Often, the theme of sacrifice is present. Maybe one of them has to make a big sacrifice for the other, and the kiss is a moment of intense emotion before or after that. It could also be about them breaking free from some kind of supernatural constraint that has been holding them back from being together, and the kiss is a symbol of their newfound freedom.
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.