In Supernatural Sam and Dean make - out fanfictions, one of the common themes is the idea of alternative universes. Writers often use the concept of different realities where the rules of their relationship are different. Another common theme is emotional turmoil. Since their relationship in the show is so firmly based on brotherhood, changing it to a romantic or sexual one in fanfiction creates a lot of internal and external conflict. There can also be a theme of transformation. Their characters may change in how they view themselves, each other, and their place in the world as a result of this new relationship.
Well, in these types of fanfictions, a common theme is often the struggle between their brotherly bond and this new romantic or sexual relationship. There's also the theme of secrecy, as they would have to keep this new relationship hidden from others in the Supernatural world. Additionally, some fanfictions might have a theme of redemption or acceptance. Maybe they initially feel guilty about their new relationship but then come to accept it. And sometimes, there's a theme of power dynamics, as they both have different personalities and abilities, and this new relationship could change how they interact in terms of power and control.