Obviously, Harry Potter himself is the central character. His journey in Slytherin would be the main focus.
One possible plotline could be Harry having to deal with the prejudices within Slytherin. He might struggle to fit in at first because of the house's reputation for dark magic, but then gradually find some true friends who see him for who he is.
Snape would have a very different relationship with Harry in this scenario. Instead of being a constant source of disdain, he might see Harry as an opportunity to shape a Slytherin in a new way. Pansy Parkinson could also be involved. She might initially be against Harry being in Slytherin but later come to accept him, or they could be in a constant battle of wits.
In such a fanfiction, Harry might face a whole new set of challenges. He could be ostracized by his Gryffindor friends at first. But he might also find unexpected allies in Slytherin. He'd have to deal with the Slytherin house's reputation for cunning and ambition in a very personal way. And perhaps he could change the perception of Slytherin from within.
There could also be other important characters like the Slytherin house - mates. Maybe a really cunning and ambitious friend who helps or hinders the main character's journey. And of course, there might be appearances from the more well - known characters from the original Harry Potter series, like Draco Malfoy, who could play a significant role in the fanfic, either as an adversary or an unexpected ally.
Obviously, Harry himself is a main character. He's the one going through the resorted situation. Dumbledore could also be a significant character as he might have some role in the resorting process or in guiding Harry afterwards.
Obviously, Harry Potter himself. Then there would probably be some Slytherin students like Draco Malfoy. Draco could play an interesting role, either as an antagonist who initially resists Harry's new - found Slytherin side or as a begrudging ally. There might also be Snape, who, despite being a Slytherin, has his own complex relationship with Harry. And of course, Harry's Gryffindor friends like Ron and Hermione, who might be a bit confused or concerned about Harry's change.
In fanfiction where Harry is resorted into Slytherin, he might face a whole new set of challenges. He could be ostracized by his Gryffindor friends at first. However, he might also find some unexpected allies in Slytherin. For example, he could befriend some of the more open - minded Slytherins who see his potential and value his skills. This new sorting could also lead to a different dynamic with Draco Malfoy. Instead of being rivals, they might become begrudging comrades or at least have a more complex relationship.
Well, a great fanfiction in this category is 'Slytherin's Secret'. In this story, Harry being in Slytherin reveals hidden secrets of the house and how he has to navigate the complex politics within it. There's also 'Harry Potter: Slytherin's Path' which focuses on Harry's journey as he makes friends and enemies in Slytherin. 'Resorted to Slytherin: Harry's Struggle' is also quite interesting as it shows his struggle to adapt to the Slytherin ideals while still holding onto his own values.
Obviously, Harry Potter himself would be a main character. Elrond, who is raising Harry, would also be a central figure. There might also be other elves like Arwen, Elrond's daughter, who could play important roles. Maybe some of Harry's friends from Hogwarts could make appearances in the story, even if it's just in his memories.
I think Dobby could also be a significant character in this fanfic. Since he has connections to both the Malfoys and Harry. He might be the one who gives the Malfoys the idea to rescue Harry or helps in the rescue in some way. Also, Voldemort or his Death Eaters could be important in the background as the threat that Harry needs to be rescued from. Their presence would constantly loom over the story, making the Malfoys' actions more dangerous and significant.