Often, the story starts with Harry being sorted into Slytherin, which is a shock to everyone. Then the abuse begins, it could be from older Slytherin students or even some of the professors who have biases. As the story progresses, Harry has to learn to use his wits, which is a Slytherin trait, to fight back against the abuse. He might also have to deal with his own internal struggle of feeling like he doesn't belong in this house but still trying to make the best of it.
Some fanfictions might explore the idea of the Slytherin's abuse being part of a larger conspiracy within the wizarding world. The Slytherin might be a key to uncovering some dark secrets, and Harry Potter, with his sense of justice, gets drawn into the mystery. This could involve political intrigue within the Ministry of Magic or among the pure - blood families. The Slytherin's story then becomes intertwined with the bigger picture of the wizarding world's power struggles.
One common theme is redemption. Harry often has to prove himself to the Slytherins who abuse him, and in the process, some of his abusers might see the error of their ways. Another theme is resilience. Harry endures the abuse and grows stronger from it, showing his Gryffindor - like courage even while in Slytherin.
One common theme is the struggle for acceptance. Harry, being in Slytherin where he is abused, has to fight to be accepted not only by his housemates but also by others in Hogwarts. For example, in many such fanfictions, he has to prove that just because he's in Slytherin doesn't mean he's like the abusers.
One common plotline is them trying to change the Slytherin house's negative image. They bring their Gryffindor values like bravery and loyalty into Slytherin and try to reform it from within. Another is their relationship with the other Slytherins. They might have to deal with the pure - blood supremacist ideas that some Slytherins hold. Some fanfictions also explore how they adapt to the Slytherin common room and its more secretive atmosphere.
One common theme is the struggle for acceptance. Harry, being in Slytherin where he might not be initially welcomed, has to fight for his place. And the abuse he faces adds to that struggle. Another theme is redemption. The romantic relationship can be a source of redemption for both Harry and the other character involved. For example, the Slytherin love interest might have their own issues and together they find a way to heal.
One common theme is redemption. Harry, having been abused, often finds a way to redeem himself or be redeemed in the eyes of others through his actions in Slytherin. Another theme is identity. He has to grapple with his new identity as a Slytherin, which is different from the typical Gryffindor image of him.
A common plot point is the Slytherin character being bullied or mistreated by other students, often from Gryffindor. For example, they might be excluded from group activities or verbally harassed. Another is the Slytherin's internal struggle with their own identity. They may feel like they don't belong in their house because of the abuse. Harry Potter might then come in as a mediator or a friend, showing that not all Gryffindors are against Slytherins.
One can start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own (AO3). On Fanfiction.net, you can use the search bar and type in relevant keywords such as 'abused Slytherin Harry Potter'. AO3 also has a great search function. You might have to filter through a lot of results, but you're likely to find some interesting stories. Some stories may focus on a Slytherin character, perhaps Draco Malfoy, who is abused in some way and how Harry Potter might be involved in the situation, either as an ally or in a more complex relationship within the Hogwarts' setting.
Perhaps there could be a plot where Harry discovers ancient Slytherin magic that only the heir can access. This could lead to him having to make difficult choices between using this power for good or being corrupted by it. He may also have to deal with the Slytherin house's expectations of him as their heir, which are quite different from what he has known in Gryffindor.
One possible plotline could be Harry using his new position to reform Slytherin house. He might change the way Slytherins are perceived in Hogwarts, promoting unity among all the houses.