There could be some original characters created by the fanfiction authors. These could be Harry's new classmates or friends at Durmstrang. They might have their own unique magical skills and personalities, and through their interactions with Harry, they can help develop the story further. For example, a new friend could introduce Harry to a secret part of the Durmstrang grounds or a special magical technique that's unique to the school.
Draco Malfoy is a very common character. Since they are in the same house in this fanfiction concept, their interactions would be different from the original story. There would likely be more opportunities for them to either become friends or rivals in a new way.
Obviously, Harry Potter himself. There would also likely be Viktor Krum, who is from Durmstrang. Krum could be either a friend or a rival to Harry. Igor Karkaroff, the headmaster of Durmstrang, would be another important character. And there could be new students created for the fanfic, like a Durmstrang student who becomes Harry's guide in the new school.
One common theme is the fish - out - of - water experience. Fem Harry is used to Hogwarts, so going to Durmstrang is a big change. Another theme is the exploration of different magical cultures. Durmstrang has its own set of traditions and ways of teaching magic that are different from Hogwarts.
Madame Maxime, the headmistress of Beauxbatons, could play an important role. She could be a mentor to Harry, guiding him in his new magical studies at Beauxbatons. She might also be involved in some sort of conflict or mystery that Harry has to help her solve, which could involve the relationship between Beauxbatons and other wizarding schools or some ancient magic within the school.
Ron could also be there. He might be a bit jealous at first if Harry's adopted family is wealthier or has more influence in the wizarding world. But later, he would support Harry as always.
There could be other Hogwarts students. For example, Neville could be there, exploring the different plants and herbs that are associated with Beltane. Luna Lovegood might also make an appearance, with her unique perspective on the magical aspects of the festival. And perhaps some local wizards and witches from the area where the Beltane celebration is taking place.
There could be a dark entity that lurks within the Durmstrang castle itself. It might have been awakened by Harry's arrival or has some connection to the school's past. This entity could manipulate students and teachers alike to create chaos and oppose Harry.
Besides Harry, Ginny could be a main character. She might be interested in the sports aspect of the Egyptian school, like their version of Quidditch. And there could be a local Egyptian student who becomes friends with Harry. This student could have a deep connection to the history of Egypt and help Harry understand the significance of certain magical sites in the country. They could explore ancient temples together and face various challenges along the way.
Hermione could still be a common character. Even in fanfiction where Harry is alone, Hermione might be mentioned in his thoughts or in flashbacks as someone who has influenced him. For example, he might recall her advice on how to study magic when he is facing a difficult spell alone.
Often, there are advisers or mentors. These are characters who have a lot of knowledge about the monarchy and its workings. They might be old nobles or wise scholars who help Harry understand the ropes of being the heir. There could also be antagonists like power - hungry nobles who see Harry as a threat to their own ambitions and try to get rid of him.