It changes Dean's character significantly. He becomes more in - touch with his emotions. Usually, he's all about the hunt and being tough. But with the mpreg, he has to deal with the hormonal changes and the fear of the unknown related to birth. He might become more nurturing, which is a big shift from his normal hunter self.
The common themes usually revolve around the shock value at first. It's not something you'd expect in the 'Supernatural' world. Then there's the theme of vulnerability. Dean, who is typically portrayed as strong and fearless, becomes vulnerable during pregnancy and birth. This can lead to themes of self - discovery for him. For John, it can be about his role as a father in this unexpected situation. There may also be themes related to the mystery of how this mpreg even occurred in the first place, whether it's some kind of magic or a curse, and how they deal with the implications of it for the 'Supernatural' world they live in.
Their relationship would likely become more complex. John would probably feel a stronger sense of protectiveness towards Dean. He might also feel guilty for not being able to prevent whatever caused the mpreg. Dean, on the other hand, might become more dependent on John in some ways, which is a big change from their usual dynamic.
One plot idea could be that Dean discovers he's pregnant and is extremely conflicted. He's worried about how John will react, as they have a complex relationship. Maybe at first John is shocked and struggles to accept it, but then realizes he has to be there for Dean. Another idea is that an enemy finds out about Dean's condition and tries to use it against them.
Another theme could be the danger and mystery that often surrounds the supernatural elements in the show, but now in the context of Dean's pregnancy. For example, there could be a plot where the cause of the mpreg is some sort of dark magic or a new, unknown entity, and Dean and the gang have to figure out how to deal with it while also ensuring the safety of the unborn child. This adds an extra layer of drama and adventure to the story.
One common theme is the exploration of Dean's character in a new and unexpected situation. The mpreg aspect forces him to face emotions and challenges he may not have otherwise.
Another place to check could be Fanfiction.net. However, the tagging system there might not be as precise as AO3. But it still has a decent number of 'Supernatural' fanfics. You may need to do a bit more sifting through the search results to find the really good Dean mpreg stories. Look for ones with high ratings and positive reviews from other readers.
One popular supernatural mpreg dean fanfiction is 'Destiny's Path'. It explores an interesting alternate universe where Dean experiences a unique situation. The story is well - written with great character development.
There are several interesting ones. For example, 'Father - Son Hunting Dynamic' which really explores the unique bond between them during hunts. It shows how they communicate, or sometimes don't communicate well. 'John's Influence on Dean' is another fanfiction that details how John's actions and decisions shape Dean's character. 'Dean's Memories of John' is a more emotional one, focusing on Dean's reminiscences of his father and how those memories impact his present actions.
There are indeed some dean x castiel mpreg fanfiction out there. For example, some stories might explore how their relationship changes with the added element of mpreg. They could deal with the challenges they face together, like how Dean copes with Castiel being pregnant and all the emotions and responsibilities that come with it. It's an interesting concept in the world of fanfiction.