It could also be a story of redemption. The female demon, despite her nature, grows to truly care for Sam. She tries to protect him from the other demons who see him as a threat. This would create an interesting dynamic as Sam is growing up in a world of demons but with a demon as his protector.
The fanfic could also explore how the demon protects Sam from other threats in the supernatural world. There are many other demons and creatures that would want to harm Sam, either because of his family name or because they sense his potential power. The female demon would have to use her own powers and wits to keep him safe, and this could lead to some exciting action scenes and tense moments in the story.
There are indeed some. One way to find them is to search on specific fanfic communities dedicated to Supernatural. These fanfics often explore different aspects of Sam and Dean's relationship. They might rewrite certain scenes from the show with a gay subtext or create entirely new storylines based on the idea of them having a romantic relationship. Just keep in mind that the quality can vary greatly from one fanfic to another.
Well, 'Magic and Monsters' is also a popular one. This fanfiction delves deep into how Hermione's magical abilities could be used in the world of Sam Winchester. It shows their journey together as they face various threats, and the character development throughout the story is quite remarkable. The author manages to capture the essence of both Hermione and Sam really well.
The theme of family legacy is quite prevalent. Since they come from a family of hunters, many fanfictions explore how this family history affects their actions and decisions. Also, the theme of growing up in a tough environment is common. They face a lot of hardships at a young age, and fanfictions often depict how they cope with it and become stronger.
One common theme is 'forbidden love'. Since Sam has his hunter duties, a relationship with an OC can be seen as something that goes against the rules of his world. Another theme is 'finding strength in each other'. The OC often brings out a different side of Sam, and they help each other grow and face challenges.
Since I don't actively engage with this type of fanfiction, I can't say for sure. But generally in fanfiction, themes might include the intense emotional connection between the characters, perhaps with a romantic or sexual undertone.
One common theme is the exploration of their deep emotional bond in a romantic context. Since they already have a strong brotherly bond in the original show, fanfictions often build on that to create a more intimate, romantic relationship.
The Archive of Our Own (AO3) is a great place to start. It has a large collection of Supernatural fanfics, including those that explore a gay relationship between Sam and Dean. The site has a good search function that allows you to filter by tags related to the characters and the type of relationship you're interested in.
One great Sam Winchester and OC fanfiction is 'The Hunter's Heart'. It has a really interesting plot where the OC is a powerful witch who teams up with Sam on a hunt. The relationship development between them is well - written.