One common theme is identity. Since Harry is in Slytherin instead of Gryffindor, he has to re - evaluate who he is. He's no longer the typical Gryffindor hero, and he has to figure out how to be a hero in his own way while being part of a house known for its cunning.
Well, in a lot of these fanfictions, a common theme is the clash of cultures. Harry coming from what is seen as a 'good' Gryffindor - type background into Slytherin. There's also the theme of secrecy. Slytherin is often associated with secrecy, and Harry might have to keep certain things about himself hidden. And then there's the theme of transformation. Harry is transformed not only by the Sorting Hat's decision but also by the experiences and relationships he has within the Slytherin house.
One popular 'wbwl slytherin harry fanfic' is 'The Slytherin's Heir'. In this fic, Harry is sorted into Slytherin and it explores his journey in a very different light compared to the original story. He forms unique relationships with his housemates and faces different challenges.
One common theme is the idea of overcoming house differences. Since Slytherin and Gryffindor are often seen as rivals, in these fanfics, the female Slytherin and Harry have to break through those barriers. For example, in many stories, they start off distrusting each other because of their house affiliations.
One common theme is self - discovery. Since Harry is a squib, he has to find his place in a magical world that he doesn't quite fit into in the traditional sense. Another theme is the relationship between Severus and Harry. Severus, as a potion master, might take Harry under his wing or have a complex relationship with him due to his status as a squib. Potions often serve as a medium through which their relationship develops or conflicts arise.
One common theme is redemption. Slytherins are often seen as the 'bad guys' in the original series, but in these fanfics, they get a chance to redeem themselves through protection. For example, they might protect a Muggle - born student they previously bullied.
One common theme is the struggle against the Slytherin stereotype. The sister character often has to deal with the perception that all Slytherins are evil or dark - wizards - in - training. For example, in many fanfics, she might be really kind - hearted but is constantly judged by others because of her house.
Well, the main plot could be about Harry being a different kind of hero in Slytherin. Maybe he uses his unique skills and the knowledge from being the Boy Who Lived (WBWL) to change the perception of Slytherin in the wizarding world. He could be on a conquest to make Slytherin a more inclusive and positive house, defying the traditional dark and cunning stereotypes associated with it.
Maybe an older Slytherin student who has been influenced by the dark arts and doesn't want Harry to disrupt the status quo in Slytherin. This student could be envious of Harry's status as the WBWL and his growing influence in the house, and thus tries to undermine him at every turn.
One common theme is survival. Since Voldemort has won, fem Harry has to find ways to stay alive in a very hostile environment. Another theme is resistance. She often becomes a symbol of hope and starts to resist Voldemort's rule. And there's also the theme of self - discovery. As she faces new challenges, she discovers new aspects of herself and her magic.
A major theme is identity. These OCs are often trying to figure out who they are within the context of the Slytherin house and Hogwarts as a whole. They might struggle with the reputation of Slytherin, or their own family legacies. For example, in some fanfics, the OC has to decide whether to follow in the footsteps of his dark - wizard family members or break free and form his own path. Also, power is a common theme. They might be vying for power within the house, or learning to use their power in a more positive way.