Family dynamics is also a big theme. How does Sam react to being an uncle? How does Castiel fit into this new family situation? And of course, how does Dean interact with the mother of the baby, if she's in the picture? These are all aspects of family dynamics that are explored.
The theme of the baby having some sort of special or mysterious quality also pops up a lot. This gives an interesting twist to the story, as the characters have to deal with not only a baby but also the implications of this special quality, whether it's a connection to the supernatural world or some other extraordinary ability.
One common theme is protection. Dean, being a hunter, often has to protect the baby girl from various supernatural threats. Another theme is family. The baby girl becomes a part of Dean's makeshift family, and it shows how he adapts to having this new family member. And love, of course. Dean grows to love the baby girl deeply, which is a central part of these stories.
Family is a very common theme. Since Dean comes from a complex family situation, fanfics often explore his relationship with his father and brother at a young age.
A theme that is sometimes explored is the baby daughter's connection to the supernatural world in a unique way. Maybe she has some sort of special ability or a link to the other side. This theme adds an element of mystery and adventure to the fanfictions. It also makes the story different from normal family - centric stories as it has that supernatural twist.
One common theme is protection. Dean, as the 'daddy' figure, is often seen protecting Sam, the 'baby prince'. This can range from protecting him from physical harm like in battles against supernatural creatures.
One great story could be 'Dean's New Family'. In it, Dean discovers he's a baby daddy unexpectedly. He has to quickly adapt to having a child in his hunter life. He tries to balance protecting the baby from the supernatural threats he faces daily and being a good father. It shows his growth from a carefree hunter to a responsible dad.
The idea of baby Dean being protected by the angels or other supernatural forces is also quite common. It often shows a different side of the Supernatural universe, with the focus on this tiny, defenseless version of Dean. And sometimes, there are time - travel or alternate - universe themes where characters from different timelines meet baby Dean.
One common theme is the struggle between good and evil. Dean often has to decide whether a vampire can be redeemed or is just pure evil. Another theme is the idea of family. His relationship with Sam and how vampires can either threaten or be integrated into their 'family' unit in a strange way. Also, there's the theme of sacrifice. Dean might have to sacrifice his own safety or even his beliefs to deal with the vampire situation.
One common theme is the idea of the oc being a powerful ally or a love interest for Dean. For example, many fanfictions have an oc with special powers that help Dean in his hunts.
One common theme is Dean's coming - of - age. It often explores his early years as a hunter and how he coped with the harsh realities of the supernatural world. Another theme is his relationships, like his brotherly bond with Sam at a younger age.