One common theme is the struggle for acceptance. Since Slytherin is often seen as the more 'ambitious' and sometimes 'dark - leaning' house in the Harry Potter universe, a pregnant Slytherin might face more judgment. So, the character often has to fight for acceptance from her peers and family.
Love and relationships are also common themes. The pregnancy can put a strain on existing relationships or lead to new ones. Maybe the father of the child is from a different house, creating a complex dynamic. Or perhaps the Slytherin character forms new friendships with unexpected people who support her during her pregnancy journey.
Another theme is friendship. Harry forms new friendships in Slytherin that he might not have had in Gryffindor. These friendships often cross the traditional house boundaries and are quite interesting to read about. For example, he might become friends with some of the more complex Slytherin characters who have their own motives and backstories. Also, the theme of self - discovery is prominent. Harry discovers new aspects of his magic and personality as he is in a different house environment.
Another theme is loyalty. He has to decide who he is loyal to - his house, his friends, or his own beliefs. In some fanfictions, he might face a dilemma where his loyalty to Slytherin clashes with his friendship with students from other houses. This creates a lot of interesting plot twists.
One common theme is the struggle against prejudice. Since Hermione is a Gryffindor in the original series, in these fanfictions as a Slytherin, she has to face the existing biases towards Slytherin house. And with her necromancy abilities, there's often the theme of the ethical use of such dark magic.
One common theme is identity. Hermione has to grapple with this new identity as the heir of Slytherin, which is very different from her Gryffindor self. Another theme could be acceptance, both from herself and from others around her.
Friendship is also a big theme. Hermione has to navigate her relationships with her old Gryffindor friends and her new Slytherin acquaintances. Sometimes there are conflicts, like when her friends don't understand her new ways. But other times, they learn to accept and even admire the new Hermione.
One common theme is identity. Harry has to re - evaluate who he is when he's a pureblood Slytherin instead of a Gryffindor. Another theme is acceptance. He has to deal with whether the other houses, especially Gryffindor, will accept him.
One common theme is the struggle between house loyalties. The female Slytherin character often has to decide between her Slytherin roots and her feelings or alliances with Potter. For example, in many stories, she may be pressured by her Slytherin peers to oppose Potter, but her own sense of right and wrong makes her question that.
In these fanfictions, the theme of transformation is quite common. Harry's entire view of himself and the wizarding world can change when he discovers his link to Salazar Slytherin. There's also the theme of mystery. Figuring out the how and why of his connection to Slytherin is often a central part of the story. Additionally, the theme of choice comes into play. Harry has to choose whether to embrace this new identity related to Slytherin or reject it, and how that choice will affect his future and the future of the wizarding world around him.
One common theme is redemption. The fanfics often try to redeem Salazar Slytherin's reputation through Harry. Since Harry is seen as a good - hearted character, if he has a connection to Slytherin, it can show that Slytherin might not have been all bad. Another theme is the exploration of power. If Harry has Slytherin's power or knowledge, how will he use it? Will he use it for the greater good like he always does, or will it corrupt him in some way?
One common theme is self - discovery. Slytherin male OCs often start out with a certain view of themselves and then through various experiences in the wizarding world, they learn more about their true nature. For example, they might realize they are not as cold - hearted as they thought.