Well, humor is also a common theme. Donna has a great sense of humor, and when combined with Castiel's often - literal and somewhat naive view of things, it creates some really funny situations. And of course, the overarching theme of fighting for good in the world full of evil is almost always present in 'castiel and donna fanfic'.
One common theme is Castiel's struggle with his angelic nature. He often has to balance his loyalty to heaven and his newfound connections with humans.
One common theme is unrequited love. In many fanfics, either Josh or Donna is secretly in love with the other for a long time without the other knowing. For example, Josh might be pining for Donna while she is focused on her career.
There can be a theme of spiritual discovery as well. In some fanfics, his blindness is seen as a way to connect more deeply with the spiritual realm. It can be a way to show that there are other ways to 'see' beyond physical sight, and it adds a really interesting layer to his character.
A common theme is the power struggle. Castiel is often seen as a more 'pure' angel, while Lucifer is the rebellious one. Their interactions are filled with this underlying battle for dominance or influence. Another theme is redemption. Sometimes Lucifer is portrayed as having a chance at redemption, with Castiel being a key factor in that.
The theme of protection is often present. Castiel, being an angel, has a natural inclination to protect. So in these fanfics, he not only babysits but also safeguards the children from various threats, whether they be physical or more supernatural in nature. For example, in some fics, he might protect the kids from demons while trying to make them eat their vegetables at the same time.
One common theme is the struggle between good and evil. Castiel is an angel, and Meg is a demon, so there's always that conflict. Another theme is redemption. Meg might be seen as someone who could be redeemed through her relationship with Castiel. And of course, love is a major theme. Their relationship, which is so unexpected, often leads to stories of love conquering differences.
Another theme is growth. In these fanfics, Castiel usually grows as a character. He might learn about human values like love, friendship, and loyalty in a more intimate way since he's part of a family. For instance, he could start to understand the concept of a family dinner and the significance of sharing time with loved ones.
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. Since both Castiel and Cassiel are often portrayed as angels, their bond of friendship is a key aspect. Another theme is redemption. Many fanfics explore how they can redeem themselves for past actions.
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.