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.
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.
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.
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.
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.
I think Darkseid would be the most unexpected. Imagine Harry Potter being raised in Apokolips. He would have to adapt his magic to survive in such a harsh and evil - dominated place. His magic could potentially be a threat to Darkseid's rule, or he could learn some dark magic - like powers from Darkseid's regime, which would completely change his character and the way he uses magic.
Hermione is often a popular crossover character. Her intelligence and magical knowledge make her a great fit in the Charmed universe. She can easily adapt to new magical situations and offer her unique perspective.
Obviously, Harry Potter and Merida would be the main characters. Harry with his knowledge of the wizarding world and his courage, and Merida with her strong - willed and adventurous spirit. They could team up to face challenges that require both magical and non - magical skills.
Obviously, Harry Potter could be one of the main characters. He could be the one to bridge the gap between the two worlds, using his courage and magic skills. Master Chief from Halo is also a great candidate. His combat experience and leadership would be valuable in the wizarding world full of new threats. Hermione would also be important as her intelligence could help in understanding the Halo technology.
Hermione Granger is quite popular too. Her intelligence and thirst for knowledge can be a great asset in the world of Eragon. She might be the one to figure out how to translate ancient dragon languages or come up with new spells by combining the two magic systems.