Well, it depends on the specific fanfiction. Some authors might make Harry the heir as part of a grand plot. For example, they could tie it to an ancient prophecy or a hidden power within the school. This can lead to new adventures and challenges for Harry, as he discovers his new role and the responsibilities that come with it.
In the fanfiction world, Harry might be the heir to Hogwarts if there was some sort of magical pact made generations ago. For example, perhaps one of his ancestors had a special role in the creation or protection of Hogwarts, and this role was passed down through the family. It could also be that his unique magical abilities, like his ability to resist Voldemort's attacks from a young age, are signs of a deeper connection to Hogwarts itself. This connection could be what designates him as the heir in the fanfiction's plot.
Hermione Granger could also be a possible Hogwarts heir. She is extremely intelligent and has a thirst for knowledge about Hogwarts' history and magic. Her Muggle - born status might seem to be a contradiction, but it could also be part of what makes her unique as the heir. She could bring new perspectives and use her vast knowledge to unlock secrets about the heir's role and the future of Hogwarts.
He might have enhanced control over the Hogwarts wards. Since he is the heir, he could be able to manipulate the protective spells around the school more easily than others.
One common plotline is Harry uncovering his heritage as the heir. Another is him facing challenges from those who oppose his new status.
One possible plot is that as the heir of Hogwarts, Harry has some special powers or abilities that he gradually uncovers. He might be able to communicate with the very essence of the school, like the walls and the grounds, and they guide him in his fight against the forces of evil. For example, the castle could show him hidden passages to outwit Voldemort's minions. Also, other characters might react differently to him once they learn of his new status. Some might be jealous, while others would look to him for leadership.
The heir themselves would be a main character, of course. And Harry Potter might still play a role, either as a friend or a rival to the heir. Dumbledore could also be a major character, guiding or observing the heir's journey.
One interesting plotline could be Harry discovering ancient magic secrets related to his status as heir. Maybe he finds a hidden chamber in Hogwarts that only the heir can access, filled with powerful spells and artifacts.
Obviously, Harry Potter himself would be the main character. Hermione and Ron would also be important as they are Harry's closest friends and would be involved in his journey as the heir.
Often, it could be dark wizards or witches who are envious of Hermione's status as the heir. They might want to take over Hogwarts themselves or steal the power that comes with being the heir. For example, a powerful dark wizard who has been in hiding for years might emerge to challenge Hermione.
Hermione is often a common character. She would be there to support Harry in his journey as the heir, using her vast knowledge of magic to help him understand the implications of his new status.