Well, first off, Ursula is an obvious main character as she is related to Harry in this fanfiction. Then there's Hagrid. He has always been there for Harry and would likely play a role in helping Harry deal with this new situation. Neville could also be an important character. He might be one of the few who initially believes in Harry and doesn't immediately turn his back on him because of his new identity. And then there are the Weasley family members. They might be a bit hesitant at first but would eventually come around to help Harry as they have always been a big part of his life at Hogwarts.
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.
The supreme Mugwump himself could be a main character. He might have his own backstory, motives, and goals that drive the story forward.
One possible main character could be a Martian elder who takes Harry Potter under their wing and helps raise him. This Martian elder could have special knowledge about the planet and its hidden magic that they pass on to Harry.
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 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.
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.
In a 'Harry Potter son of Ursula' fanfiction, the plot could revolve around Harry trying to figure out how he could be Ursula's son. It might start with some strange events or revelations. He could face a lot of rejection from his friends at Hogwarts as they are suspicious of his new identity. He then has to go on a journey to find out the truth about his origin and clear his name. He might also encounter some new magical creatures or spells along the way.
One potential Quidditch superstar in fanfiction could be a new transfer student. This student could have been a star player in their previous school's Quidditch team and brings a whole new set of skills and strategies to Hogwarts. Maybe they have a unique flying style that no one at Hogwarts has seen before.
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.