Love and protection are also big themes. Castiel is often portrayed as a very protective character. In these fanfictions, he will go to great lengths to protect his pregnant girlfriend. And the love between them is usually shown as a strong force that can overcome any difficulties, whether it's supernatural threats or just the normal hardships of pregnancy and relationships.
One common theme is the challenges they face together. Since Castiel is an angel in the Supernatural universe, there are often issues related to his angelic nature and how it affects the pregnancy. For example, his powers might have unexpected effects on the unborn child or the mother's health.
Another theme is the reaction of the other characters. In the Supernatural world, Castiel has friends and foes. His friends like Sam and Dean might have different reactions to his girlfriend being pregnant. Some might be supportive, while others might be worried about the implications for their mission or the safety of everyone involved. And of course, his enemies might see this as an opportunity to target him or his loved ones.
One common theme is identity. Since Castiel is usually male in the original series, when she's female in fanfiction, there's a lot of exploration about how she sees herself and how others perceive her. For example, some fics show her struggling with the new body and the expectations that come with it.
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 redemption. When Castiel is hurt, often the story will revolve around him finding a way to redeem himself, whether it's from a mistake he made that led to his pain or just from a sense of lost purpose. For example, in some fanfictions, he might have been hurt because he trusted the wrong entity and then has to make amends.
One common theme is the theme of rescue. Usually, the other characters, like Dean and Sam, will go to great lengths to save Castiel. For example, they might face numerous dangerous creatures or powerful enemies in their quest to rescue him.
One common theme is redemption. Since Castiel has made some mistakes in the show, a lot of fanfiction focuses on him trying to make up for those. Another theme is his relationship with the other main characters, like Dean and Sam. His loyalty and how it is tested is often explored.
One common theme is Castiel's struggle with his angelic nature and his growing humanity. He often has to choose between following angelic orders and doing what he believes is right. Another theme is his loyalty to the Winchesters, especially Dean. His relationship with Dean is a central focus in many fanfics, whether it's a deep friendship or something more. Also, themes of redemption are common, as Castiel has made many mistakes in the series and fanfics often explore his path to redemption.
You can try searching on fanfiction platforms like Archive of Our Own (AO3). It has a vast collection of fanfictions, and you can use the search function to specifically look for 'Castiel pregnant girlfriend' fanfictions. There are also other platforms like FanFiction.net, but AO3 is known for its wide range of works and tagging system that can help you find exactly what you're looking for.
The theme of unrequited love can be common. Maybe Castiel has feelings for someone who doesn't return them. Guilt is also a big one. He might feel guilty for actions he took that had negative consequences. And the idea of being an outcast. Since he's different from both angels and humans in many ways, he often feels like an outcast, which brings about a lot of sadness in fanfictions about him.
One common theme is family. Since Castiel is the little brother, stories often explore his relationships with his siblings, like loyalty, rivalry, or protection within the family. Another theme is growth. As the little brother, he is likely to grow and develop, both in power and character. For example, he might learn important lessons about himself and his place in the family. Also, discovery is a theme. He could discover new abilities or secrets related to his family or his own identity.
Isolation is a very common theme. Since Castiel is abandoned, he often finds himself alone, cut off from his former celestial family. This isolation makes him question his identity and purpose.