He could meet some of the Hogwarts ghosts. They might have some interesting stories or information about the wards that could help Harry in his task.
He might run into some of his old professors like McGonagall. She would be glad to see Harry back at Hogwarts. And perhaps he'll also meet some new students who look up to him as a hero. These new students could ask Harry for advice on how to be brave in the face of danger and how to master difficult magic spells.
In the fanfic, Harry Potter could meet the new generation of students. They would be in awe of him. He might also encounter some of the ghosts, like Nearly Headless Nick, who would surely have some interesting stories to share since Harry left. And there's a good chance he'll meet the caretaker, who would probably give him a hard time about coming back and causing a stir. Moreover, he could cross paths with some of the older students who idolize him and want to learn from his experiences.
He could meet Hagrid. Hagrid often goes to Diagon Alley, so it's possible he'll run into Harry while he's shopping and give him some useful advice about what to buy or not to buy.
One consequence could be that it alerts all the magical defenses of Hogwarts. This means that every protective spell, charm, and enchantment within the castle would be on high alert. For example, the portraits might become more vigilant, the secret passages might close off more tightly, and the ghosts could be more active in patrolling the corridors.
He could meet centaurs. Centaurs are known to live in the Forbidden Forest in the Harry Potter universe, and if Harry is living in the woods, it's very likely he'd cross paths with them. They might teach him about astronomy and prophecy in a more in - depth way than he learned at Hogwarts.
In some fanfictions, it could be by accident. Maybe he casts a spell that interacts with the wards in an unexpected way. For example, if he tries a new spell he learned outside of Hogwarts and it has a frequency that matches the activation code of the wards.
In some fanfiction, the Hogwarts ancient wards are described as a complex web of magic. They are like invisible lines of power that crisscross the entire school grounds.
In Harry Potter fanfiction, the Hogwarts ancient wards can serve many functions. For example, they can protect the school from dark magic. They act like a magical shield that keeps out evil forces. Also, they might be used to prevent unauthorized access to certain areas of the school, like the Forbidden Forest or the Chamber of Secrets.
In fanfiction, if Harry controls the Hogwarts wards, it might also change the power dynamics within the school. Teachers and students would have to adjust to this new authority. He could potentially use the wards to create safer areas for students to practice magic without fear of being attacked. This could lead to a more confident and magically proficient student body.
Another possibility is that his connection with the Sorting Hat plays a role. The Sorting Hat has a deep knowledge of Hogwarts and its magic. If Harry were to have a special conversation with the Sorting Hat or if the Hat were to bestow some sort of power upon him, it could enable him to activate the wards. After all, the Sorting Hat has been known to recognize and respond to the qualities and needs of the students at Hogwarts.