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.
Another interesting one is 'Dean's Little Prince Sam'. This fanfiction explores the idea of Dean seeing Sam as his precious 'baby prince'. It has many touching moments where Dean goes to great lengths to make sure Sam is happy and safe. The story also has some great character development for both of them.
One common theme is protection. Dean is often portrayed as extremely protective of Sam. For example, in many stories, he will go to great lengths to keep Sam safe from any harm.
One common theme is family. The baby bump represents a new addition to their family, and it often shows how Dean and Sam redefine their idea of family. Another theme is protection. Dean is usually very protective of Sam when he has the baby bump, which shows their strong brotherly bond.
One common theme is redemption. Demon Dean often has a chance to redeem himself through his relationship with baby Sam. Another theme is protection. Dean, even as a demon, feels the need to protect Sam. For example, in many fanfictions, he'll fight off other demons who try to harm Sam.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of 'dean and baby sam fanfiction' stories. Just use the search bar and type in relevant keywords like 'dean and baby sam' and you'll get a bunch of results.
One great 'dean and baby sam fanfiction' is 'The Early Days'. It beautifully portrays Dean's protective nature towards baby Sam. Another is 'Sam's First Steps in Dean's Care' which focuses on the little moments of Sam growing up under Dean's watchful eye. And there's also 'Dean and Sam's Bond from the Start' which delves deep into their emotional connection right from when Sam was a baby.
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.
Sam would probably be very protective. He has always been there for Dean, and now with a new addition to the family in the form of a baby, he would see it as his responsibility to keep the baby safe. He might also be a bit of a mentor, teaching the baby about the world of hunting and how to handle different supernatural creatures.
One great 'dean and sam baby bump fanfiction' is 'A New Beginning' where it beautifully explores the idea of Sam expecting a baby. It shows how Dean supports him through the process and the changes it brings to their relationship.