A very common one is Sherlock Holmes. Imagine Harry being raised by the brilliant detective. He would grow up with a sharp mind for deduction, which could be very useful in solving the mysteries within the wizarding world. Another is Gandalf from 'The Lord of the Rings'. Gandalf could teach Harry a different kind of magic and wisdom, which would shape Harry into a very different kind of wizard.
Severus Snape is also a popular choice. His complex character and vast knowledge of magic can create interesting dynamics when he meets characters from different magical worlds. He could either be a mentor or an antagonist in these crossovers.
Often, Harry Potter himself is a central character. He might interact with immortals and be influenced by their ways. Hermione could also be there, using her intelligence to understand the immortals and their magic. Voldemort could be another common character, and his quest for power might be affected by the presence of immortals. Maybe he tries to gain immortality from them or is in conflict with them because they pose a threat to his plans.
In many 'harry potter horror crossover fanfiction', Sirius Black can be a key character. His time in Azkaban and his connection to the dark arts (through his family) can be used to create horror - inspired storylines. Luna Lovegood might also be included as she has an open - minded and somewhat otherworldly view. This could lead to her being the first to sense the encroaching horror elements in the wizarding world, like strange new creatures or dark magic manifestations.
One possible plotline could be Harry being raised by characters from a different magical universe, like the Narnia characters. This might lead to him having a very different understanding of magic and his role in the wizarding world. Another could be if he was raised by characters from a superhero universe, which would likely result in him having some unique combat skills in addition to his wizardry.
Edward Cullen and Harry Potter are often popular. The contrast between Edward's vampire nature and Harry's wizardry makes for an interesting dynamic. Fans like to imagine how they would react to each other's powers and the situations they face.
The theme of identity is also prevalent. The fem Harry Potter has to figure out who she is in a world that has certain expectations. With the crossovers, there are additional factors that influence her identity. She might have to balance her Hogwarts self with the new elements introduced from the other fictional universes she is crossed over with.
Redemption can be a theme too. Some fanfics might have a creature that was once seen as evil or dangerous, but through its interactions with the characters in the Potter world, it gets a chance at redemption. This could be a werewolf finding a way to control its transformation and be accepted, for example.
Power dynamics are also common. Since he's submissive, there are often power struggles where he has to either submit to a more powerful entity or find ways to overcome being in a submissive position. This can involve political intrigue in the crossover world, like in a crossover with a fantasy kingdom where there are complex power hierarchies. And also, themes of self - discovery are present. As Harry is in a submissive role, he learns more about his own limits and capabilities, which is a journey of self - discovery.
Another theme is the clash of cultures. For example, if Harry is adopted into a non - magical world like a superhero universe, there will be a contrast between the magical and non - magical ways of life. It's about how Harry and the other characters adapt to this difference. For instance, in a crossover with the Marvel Universe, Harry has to learn about modern technology and superhero ethics while still having his own magical abilities and values from the wizarding world.
One common theme is the clash of different magical systems. Harry Potter has its own brand of magic with wands and spells, while Buffy has slayer powers and a different kind of magic related to demons and the supernatural. Fanfictions often explore how these two types of magic interact when the characters meet.