There could also be some new characters introduced. Maybe a student who also had a late start in a previous year and can give Harry some advice. Or a magical creature that Harry encounters on his way to Hogwarts late and that creature becomes some sort of guide or protector for him during his time at Hogwarts.
One possibility is that Harry's late arrival at Hogwarts could change the dynamics of the relationships he forms. For instance, instead of being in the same boat as the first - years from the start, he might end up befriending some older students who take him under their wing. Also, his late start could mean that he discovers parts of Hogwarts that others haven't yet, like secret passages or rooms that are only accessible to those who arrive late or are in a certain situation.
His family members also play roles. His father, Harry Potter, could be mentioned, perhaps giving Albus advice or being a source of inspiration. His siblings might also be included, adding family - related sub - plots to the story.
It could be an old, kind - hearted Muggle woman who has seen a lot in her life. She might have a small office filled with plants and cozy furniture. She takes Harry in as a patient because she senses his deep - seated sadness. Even though she doesn't know about the wizarding world, she has a natural intuition for helping people.
Harry Potter himself could be one of the main characters. He has a natural inclination towards heroism and protecting others, so he would fit well as a Hogwarts Guardian.
He could meet the Dalton students. They might be very different from his Hogwarts friends, maybe more focused on academic magic in a different way.
He could meet an Egyptian wizard who is an expert in ancient magic. This wizard could teach Harry a great deal about the history of magic in Egypt.
It could be a new dark wizard who has been lurking in the shadows. Maybe someone who was once a student at Hogwarts but turned to the dark side.
The main antagonists could be those who don't want Harry to receive a proper Hogwarts education. Maybe some dark wizards who fear that Harry will become too powerful if he goes through the Hogwarts training. So, they plot to keep him away from Hogwarts.
Hermione would surely be a main character. She could be a fellow teacher, perhaps teaching Transfiguration or History of Magic. She would be the one Harry goes to for advice on teaching methods or when he has a difficult student. Another character could be Ron. He might visit Harry at Hogwarts and end up getting involved in some of the school's mysteries or helping Harry with a particularly difficult Quidditch - loving student.
Luna Lovegood could be a main focus. Her unique perspective on magic could be used to discover new magical phenomena that emerged after the battle. She might also be helping the other students deal with their post - battle stress, using her calm and dreamy nature to soothe them.