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.
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.
Another idea could be Harry learning magic from a time - traveling wizard from a far - off future. This would introduce futuristic magical concepts and technologies that he would have to adapt to. It could also lead to him having to use his traditional Hogwarts magic in new and unexpected ways to keep up with the advanced magic he's learning.
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.
Obviously, Harry Potter is a main character as the title suggests. He is the one who is on the journey of learning wandless magic. Hermione Granger could also be an important character. She might be helping Harry research about wandless magic, using her vast knowledge from the library.
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.
In a Harry Potter and LOTR crossover fanfiction, black magic could take on different forms. In the Harry Potter universe, it's often associated with the Dark Arts like the Unforgivable Curses. In LOTR, dark magic is related to Sauron and his minions. A crossover might explore how wizards from Harry Potter's world react to the more ancient and malevolent black magic of Middle - earth.
Besides Harry, there could be a character who is a descendant of an ancient elemental magic family. This character might initially be a rival to Harry in mastering the magic but later becomes an ally. Also, Dumbledore could play an important role. He might be secretly guiding Harry in his journey of learning elemental magic, using his own deep knowledge of the wizarding world's secrets.
Hermione is often a common character. She might help Harry research and understand the ancient magic, using her vast knowledge from the library. Ron could also be there, providing his loyalty and sometimes comic relief during Harry's journey of learning ancient magic.
Snape is a character that could play a major role. Given his past with dark magic, he might be a reluctant mentor to Harry, guiding him in understanding the power and danger of it. Dumbledore, too, would likely be in the story, as he is always concerned about Harry's well - being and actions.