Well, if Sirius was in Slytherin, his relationships with the other characters might have been very different. He could have struggled to find true friends or might have been influenced by darker magic.
If Sirius was sorted into Slytherin, it's likely that his experiences at Hogwarts would have been much more challenging. He might have faced more suspicion and prejudice, and his path to becoming a hero could have been a lot more complicated.
One possible fanfiction could be about Sirius Black being in a situation where he has to face Voldemort alone. Maybe he discovers a secret weakness of Voldemort that no one else knows. Sirius, with his brave and reckless nature, decides to take advantage of this knowledge to try and take down the Dark Lord once and for all.
It could lead to a completely different character arc for Peter. Maybe he'd become more cunning and power-hungry in Slytherin.
Another plot could involve Harry discovering some long - lost Slytherin secrets. Maybe there are hidden chambers or spells that only Slytherin students are supposed to know about. Harry, being the curious and brave character he is, delves into these secrets. This could put him in a dangerous position as some Slytherins might not want him to expose or use these secrets, but it also gives him a chance to prove himself to his new housemates.
One interesting plotline could be Hermione struggling to fit in Slytherin at first. She's used to the values of Gryffindor, so being in Slytherin where ambition and cunning are prized in a different way would be a big adjustment. For example, she might have to deal with the initial suspicion from her Slytherin housemates who are not used to having a 'Mudblood' among them. But as time goes on, she could use her intelligence to show them that she belongs there and maybe even change some of their views on blood purity.
Well, he might face a lot of challenges. In Slytherin, he'd be surrounded by students with different values compared to Gryffindor. He could have a tough time making true friends at first. But it might also give him a chance to change the perception of Slytherin from within. He could use his Gryffindor courage to influence the more cunning and ambitious Slytherins towards a more positive path.
One common trope is the outsider - within - Slytherin. Hermione is smart and different from the typical Slytherin, so she has to find her place. Another is the redemption of Slytherin as a whole. With Hermione there, she might start to change the house's more negative aspects. There's also the friendship - turned - rivalry - turned - friendship again with characters like Draco. They start off on the wrong foot, become rivals, but then realize they can work together.
Well, usually it leads to a lot of interesting plot twists and character developments. Maybe Snape has a hidden agenda or Hermione discovers new sides of herself in Slytherin.
If Hermione was sorted into Slytherin, she'd bring a whole new dynamic to the house. Slytherin is known for its ambition and resourcefulness, and Hermione has those traits in spades. She might change the perception of what it means to be a Slytherin. Instead of just being about pure - blood supremacy and cunning for the sake of it, she could introduce a more ethical form of ambition. Her study habits would probably influence some of the other Slytherins to be more studious. And she'd also have to navigate the politics within Slytherin, which could be a real challenge as she's used to the more straightforward environment in Gryffindor.
One idea could be Harry having to deal with the prejudice from his Gryffindor friends. For example, Ron might be initially very suspicious and distant, which forces Harry to prove that he's still the same person. Another plot could involve Harry using Slytherin's more cunning and ambitious traits to fight Voldemort in a different way than in the original story. He might form alliances within Slytherin that were previously unthinkable.
There's a fanfic titled 'Harry in Slytherin: Overcoming Abuse'. In it, Harry's sorting into Slytherin is a shock to many. But within Slytherin, he finds a sense of belonging. The house's values of ambition and self - preservation help him deal with his abuse. The story shows how he transforms his pain into strength with the support of his Slytherin peers. It's a story of redemption and growth.