Some of Harry's friends could also be antagonists in this fanfiction. They might not understand his reasons for wanting to destroy the wizarding world and try to stop him. For example, Hermione, with her love for knowledge and the wizarding world's institutions like Hogwarts, would probably try to reason with Harry and, if that fails, might even fight against him to protect the world she loves.
It could also be a story where Harry has lost all hope. He has seen too much death and destruction in the wizarding world, from the war with Voldemort. He might think that the only way to end all the pain and suffering is to just end the wizarding world as it is. He could start by attacking the key institutions like Hogwarts and the Ministry of Magic.
One possible reason could be the trauma he has faced. Harry has endured a great deal of pain, from the loss of his parents to the constant battles and threats in the wizarding world. He might want to escape all that and find peace away from it.
There would be shock and disbelief. The wizarding world has always seen Hogwarts as a symbol of hope, knowledge and safety. If Harry destroys it, many would be in denial at first, thinking it couldn't be true. Wizards and witches from all over would come to the ruins, trying to figure out what had happened.
It could be that in the fanfic, Harry Potter discovers some deep - seated secrets about the wizarding world's power structure. Feeling betrayed, he decides to take extreme actions to expose and destroy it. For example, if he finds out that the Ministry of Magic has been using unethical means to control the wizarding population, he might use his influence and magic to bring it all crashing down. He could start by revealing the truth to the public, which could lead to chaos and the destruction of the established order in the wizarding world.
A 'Harry Potter World Traveler' fanfic can expand the wizarding world in multiple ways. Firstly, it can introduce new magical locations. Instead of just Hogwarts and Diagon Alley, we get to see enchanted cities in South America or hidden islands in the Pacific. Secondly, it can explore different forms of magic. There could be a type of magic based on music in some far - off land. And thirdly, it can bring in new characters from various parts of the world, each with their own backstories and motives.
It could be about Harry Potter achieving ultimate power and ruling the wizarding world. Maybe he overcomes new and even more dangerous dark forces and in the process, he becomes the king. He could make new laws and bring about great changes to Hogwarts and the entire wizarding community.
Obviously, Harry Potter himself. Hermione and Ron would most likely be there too, as his closest friends. Dumbledore might still play a role, perhaps as an advisor to Harry in his new position. And there could be new characters, like wizards and witches from other parts of the world who come to see the new king.
One possible reason could be that he's tired of the constant danger and politics in the wizarding world. After all he's been through so much. He might just want a normal, quiet life away from magic.
One possible reason could be extreme anger. If something really bad happened to his loved ones, like if Voldemort came back in a different way and killed all his friends and family this time, Harry might be driven to the edge and decide to destroy the wizarding world that couldn't protect them.