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.
Dumbledore would likely be a character in these fanfictions. He would probably be aware of Harry's adoption all along and be watching over him from afar, guiding him in the shadows to ensure he grows up to fulfill his destiny as a powerful wizard.
The adoptive family members could be main characters. For instance, the adoptive parents who might have their own unique personalities and backstories. They could be former Aurors who retired early to focus on raising Harry. There could also be an older sibling in the adoptive family, who might initially be jealous of Harry but later becomes his closest ally.
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.
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.
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.
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.
Hermione is often a common character. She would probably be by Harry's side, using her vast knowledge to help him understand his role as the Gryffindor heir. Ron might also be there, providing his loyalty and comic relief in tense situations.
Ron could also be there. He'd bring his sense of humor and loyalty. Maybe in a story set in Australia, Ron would be the one to make friends with the local magical folk first because of his easy - going nature, and then introduce Harry to them.
The Weasleys are very common. Given their big hearted nature and their closeness to Harry in the original story, it makes sense for them to adopt him in fanfiction. They already treat him like family, so it's a natural fit.
Obviously, Harry Potter himself is a central character. Hermione and Ron might also play important roles. They could be trying to figure out what happened to Harry and how to reverse the deaging. Dumbledore might also be involved, perhaps guiding the situation from the shadows or trying to protect Harry in his new situation.
Ginny Weasley could be another character. Given her relationship with Harry in the original series, in the fanfiction she might be part of the harem, bringing her own strong - willed and talented self into the mix, and her experience with the Weasley family's magic and values could influence the group.