It could be that there's a powerful enemy that has taken over Hogwarts and Harry, in a desperate attempt to stop this evil from spreading, resorts to destroying the school. He might think that rebuilding it later is better than allowing it to be used for evil purposes. For example, if Voldemort had found a way to permanently taint the magic of Hogwarts, Harry might see destruction as the only option.
One possible reason could be a dark turn in the story. Maybe Harry is under the influence of a dark magic or a powerful and evil entity that makes him lose control and start to destroy Hogwarts. For example, if he was possessed by a powerful dark spirit that wanted to eradicate the symbol of the wizarding world's good, which is Hogwarts.
It could be a new dark wizard we haven't seen in the original series. Maybe someone who has a vendetta against Hogwarts and manipulates Harry into doing the dirty work of destruction.
Well, perhaps Harry discovers some dark secrets within the Ministry that are so evil he decides to take it down. Maybe they're involved in some sort of illegal dark magic experiments or corrupt deals with dark wizards.
One possible plot could be that Harry leaves Hogwarts to hunt down the remaining Death Eaters on his own. He might feel that he can't put his friends in danger anymore and takes it upon himself to rid the wizarding world of the last remnants of Voldemort's followers.
I'm not sure about the specific plot of this fanfiction as it sounds rather unorthodox. It could be a very dark or alternate - universe take on the Harry Potter world where Harry goes against the Weasleys for some extreme reason.
One possible reason could be that he was under the influence of a powerful dark magic or a curse that made him lose control. Another reason might be that he was tricked into believing Hogwarts was a threat to the wizarding world in some extreme way. Maybe a powerful enemy manipulated him to think that destroying Hogwarts was the only way to save everyone.
Yes, it is possible. Fanfiction allows for creative reinterpretations of established universes. In this case, imagining Harry Potter as the founder of Hogwarts could lead to some really interesting storylines. It could explore how his modern - day knowledge and experiences would shape the school in its earliest days, perhaps introducing new spells or ways of teaching that are different from what we know of the original founders.
Perhaps in the fanfiction, there is a prophecy that Snape will cause great harm to Hogwarts. Harry, being the hero, feels it's his duty to stop Snape. He starts to gather evidence of Snape's wrongdoings, which are not obvious at first. As he digs deeper, he uncovers a web of lies and deceit that Snape has spun. This leads to a confrontation where Harry uses his magic skills and the loyalty of his friends to destroy Snape's influence over the school and the other wizards. Snape might have been planning to use dark magic to take over Hogwarts, and Harry is the only one who can prevent it.
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.
One possibility could be that Harry discovers some dark magic or a hidden conspiracy within the Triwizard Tournament that forces him to take extreme measures and destroy it. Maybe he finds out that the tournament is being used as a cover for an evil plot against Hogwarts or the wizarding world in general.