Dumbledore could play an important role too. He might be the one trying to figure out what spell caused Harry's transformation and how to reverse it. He could also offer some wise words to Harry about identity and how the form doesn't really change who she is on the inside, that her courage and magic are still the same regardless of being a boy or a girl.
Ron could also be a major character. He might have a hard time accepting Harry as a girl at first. But as the story progresses, he could become more understanding and protective of Harry in this new form. And there could be some really funny and awkward moments between them.
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.
One of the main characters could be a Protoss high templar. This character could be the one who first discovers Harry and decides to take him under his wing. Another could be a Protoss zealot, who is initially skeptical of Harry but later becomes his ally in battles.
His family members might also be important characters. His parents, if they are alive in the fanfic, could be the opposite of Harry when it comes to snow. They might be the ones who want to take family pictures in the snow every year, and Harry dreads those moments. Or a younger sibling who is always excited about snow and can't understand Harry's dislike.
Hermione could be one of the main characters. She might be suspicious of Orochimaru's teachings and try to convince Harry that it's wrong. She would be constantly researching about Orochimaru and the type of magic he's teaching Harry to prove her point.
Hermione could be one of the main characters. She might be the one who tries to find Harry and bring him back. She would use her intelligence and knowledge of magic to track him down, all while worrying about his safety.
Rainbow Dash might also be a main character. She's brave and loves adventure. She could take Fem Harry on some wild rides around Equestria and introduce her to the exciting parts of the world. They might even get into some friendly competitions.
An older, wiser mentor figure could be included. This person has seen the world and can offer Harry advice on how to grow out of his playboy phase. They could have a backstory of their own, like they were once a playboy or knew someone who was and learned from their mistakes. For example, he could be an old actor who has retired from the limelight and now wants to help the younger generation.
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.
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.
The members of Wildwind would surely be important characters. They could interact with Harry, maybe give him backstage passes or even ask him for his opinion on their new songs.