There could be a female leader on the island. She might be strong - willed and initially resistant to Harry's presence but later comes to respect him.
Perhaps there's a young girl on the island who is curious about Harry and his different ways. She could be the one who helps Harry understand the island's culture better. And then there might be an older woman, like a healer on the island, who uses her knowledge of local herbs and remedies to help Harry when he gets injured or sick.
Maybe some of the Hogwarts professors could be involved. For example, Professor McGonagall could be concerned about Harry's new situation and offer her guidance. There could also be some descendants of previous island owners or caretakers who have their own interests in the island. They could either be friends or foes to Harry, adding more complexity to the story. Additionally, some magical creatures native to the island could be almost like characters themselves, interacting with Harry in unique ways.
Dumbledore could be a character in the fanfiction. Maybe he knew about the island all along and left it to Harry in his will. Or he could be a guiding spirit for Harry as he figures out what to do with the island.
Hermione and Ron would likely still be main characters. Hermione would use her intelligence to help Harry understand his new identity and its implications. Ron, with his loyalty, would stand by Harry through any difficulties that come his way. There could also be Death himself, who might make appearances to guide or test Harry. And perhaps Dumbledore, as he always has a role in Harry's life in the wizarding world, could be involved in helping Harry navigate this new revelation.
Hermione would surely be involved. She could be Harry's research partner, helping him find relevant medical information in books and scrolls. Her intelligence and thirst for knowledge would be very useful in exploring new areas of medical magic. Another character could be Madam Pomfrey. She could be Harry's mentor, teaching him the basics of wizarding medicine and guiding him in his new role as a medic.
Hermione would surely be there. She might be a fellow professor, teaching a subject like Transfiguration or Ancient Runes. She could be Harry's confidante and help him with academic matters at Hogwarts. Also, Ron could make an appearance, perhaps as a visiting lecturer from time to time, sharing his knowledge of wizarding chess and other aspects of wizard culture.
His child, of course. Depending on the story, the child could be a young boy or girl. If it's a boy, he might be eager to learn magic from his father and get into all sorts of mischief at Hogwarts when he's old enough. If it's a girl, she could be more interested in the magical creatures and want to follow in Hagrid's footsteps.
Hermione would most likely be one of the main characters. She's always observant and caring towards Harry. So when she notices something is off with Harry, she would do everything in her power to figure out what's wrong and help him. She might use her extensive knowledge of magic to find a solution to his emotional distress.
The dragon, of course, is a major character. It could be named something unique like 'Fyrstorm'. There might also be Hermione and Ron. Hermione would probably be worried about the legality and danger of raising a dragon, while Ron might be both scared and a bit excited about it.
Hermione could be a main character. She might help Harry understand the implications of being heir to multiple houses. Ron could also be important as he stands by Harry through all the new revelations. There could also be characters like Draco Malfoy. If Harry is heir to Slytherin, Draco's attitude towards Harry might change significantly, adding an interesting dynamic to their relationship.
Hermione could be a major character. She might use her intelligence to try and figure out where Tom has taken Harry. She could be researching old spells or looking through Tom's past hideouts to find clues.