The Queen (if it's a modern - day or alternate - reality Britain with a monarchy) could be a character. She could be the one who officially sanctions the efforts to bring Harry back, seeing him as a symbol of hope and unity for the nation. Her influence and power could be used to mobilize resources and people in the quest to get Harry back to Britain.
Ron could also be a major character. He and Harry have a long - standing friendship. Ron might be torn between his loyalty to Harry's new life away from Britain and his own love for his homeland. His perspective could add an interesting layer to the story as he tries to balance both.
One possible plot could be that after some time away, Harry has gained new skills or knowledge. Britain is facing a great threat, perhaps a magical one if it's in a wizarding world context. The people realize that Harry is the key to solving the problem, so they try to convince him to come back. They might send his old friends to persuade him, and he has to decide between his new life and his old homeland that now needs him.
One of the main characters could be a goblin named Griphook. He might play a crucial role in helping Harry understand the significance of the artifacts and guiding him on how to return them properly.
Hermione could be one of the main characters. She would be very helpful as she is smart and well - read in magic. She could offer advice on the best spells to use for building the castle and how to make it structurally sound.
Hermione would definitely be one of the main characters. She would be trying to research a way to either reverse Harry's transformation or help him control his werewolf side. Ron might also be there, at first a bit scared but then standing by Harry's side.
Hermione could be a main character. She might try to figure out why Harry hates himself and be the one to pull him out of that dark place. Ron could also be there, maybe at first not understanding Harry's self - hatred but later becoming a source of support. Dumbledore could be another key character, as he often has insights into the students' minds and could play a role in helping Harry deal with his self - hatred.
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.
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.
Voldemort would surely be a main character. With Harry dead as a baby, Voldemort's actions and how he further consolidates his power would be a major part of the story.
There could be new characters introduced in this fanfiction. For example, a local wizard in the place where Harry moves to could become a main character. This wizard could initially be suspicious of Harry but later become his friend. Also, a magical creature that Harry befriends in his new location could be a central figure. It could help Harry adapt to his new surroundings and teach him things about the local magic that he didn't know before.
Dumbledore could also play a role even if Harry has left Earth. Maybe his spirit or some form of his magic reaches out to Harry from afar, guiding him on his journey and providing him with cryptic advice to face the challenges in outer - space - related magic.