The witches are often popular. They have a lot of powers and can be both good and bad, which gives a lot of room for character development. For example, a witch might start off using her powers for selfish reasons but then learn to use them for the good of the community.
Vampires are also very popular. Their eternal life and the whole blood - drinking aspect make for interesting stories. There are often stories about vampire families trying to keep their existence a secret or about a lone vampire trying to find their place in the world of Sims.
Castiel is also a favorite. His angelic nature and his complex relationship with the Winchesters can take on new dimensions in inflation fanfiction. For instance, if there's an inflation of angelic powers, how he navigates that while still being involved with Sam and Dean makes for great story material.
Dean Winchester is quite popular to be genderswapped. His strong personality and complex backstory make for an interesting female version. People like to explore how a female Dean would interact with the other characters, especially Sam. Sam Winchester is also a common choice. A genderswapped Sam can bring new perspectives to the story.
Abaddon is often a popular 'Knight of Hell' character in fanfiction. Her strong and devious nature makes for great storytelling. Writers can explore her backstory more deeply, like her rise to become a Knight of Hell and her motives for causing chaos in the Supernatural world.
Sam and Dean Winchester remain popular as well. Their relationship with Chuck, whether it's one of loyalty, betrayal, or a complex mix, is often explored. They are the main protagonists of the original series, and in these fanfictions, their reactions to Chuck's true identity are key elements.
Batman is definitely a popular choice. His detective skills would be very useful in dealing with supernatural mysteries. He could use his knowledge of Gotham's history and its hidden secrets to figure out what's going on when faced with supernatural threats.
Issei Hyoudou is often a popular choice. He's the main character in the original series, so there's a lot of material to draw from and expand on. Writers can explore his growth as a character, his relationships with other characters, and his development of new powers.
In some of these fanfictions, Ruby can be a popular character. She has her own history with the Winchesters and with the underworld. When a daughter of hell is introduced, Ruby's knowledge and her own motives can make for some interesting plot developments. And then there's also Rowena, a powerful witch. Her magic and her devious nature can create all sorts of trouble and alliances when combined with the presence of a daughter of hell.
Sam and Dean Winchester are always popular in Supernatural fanfiction, including those based on 13x17. Their brotherly bond and their roles as hunters are often central to many stories. Castiel is also a favorite. His angelic nature and his relationship with the Winchesters make him a great character to write about in relation to the events of 13x17.
Sam Winchester is quite popular to pair with Lucifer. Their connection in the show, with Sam being one of Lucifer's vessels, makes for an interesting dynamic in fanfiction. Also, Castiel. The angel's moral compass and his power contrast well with Lucifer's nature.
Probably the main characters from the original work tend to be popular in fanfiction. They are well - known and have established personalities that writers can build on.