Guilt is another major theme. Castiel is usually ridden with guilt for his actions towards Dean, and this guilt drives much of the story's emotional arc. For example, in many stories, we see Castiel isolating himself as he can't bear to face Dean after what he's done.
The theme of forgiveness also plays a role. Dean has to decide whether to forgive Castiel or not. This can be a long and complex process, often involving a lot of soul - searching on Dean's part and many attempts at reconciliation from Castiel. It really shows the complexity of their relationship in these fanfictions.
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 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.
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 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.
Guilt is a very common theme. Since Sam and Dean have a strong bond, when Sam hurts Dean, he often feels intense guilt which drives the plot forward.
You can start by looking on Archive of Our Own (AO3). It has a vast collection of fanfiction, and there are likely many 'dean hurts castiel' stories there. Fanfiction.net is also a good place to check.
One great 'dean hurts castiel fanfiction' could be 'The Broken Bond'. It delves into the emotional turmoil Dean experiences when he accidentally causes harm to Castiel. Another is 'Healing the Wounds' which focuses on how they both overcome the pain and strengthen 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.