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.
There's also a chance that the antagonist is an inanimate object or a force within Hogwarts itself. For example, a cursed part of the castle's foundation that spreads its evil influence, making Harry believe that the only solution is to destroy the whole place to get rid of the curse.
Perhaps it's a corrupted version of Dumbledore. In a dark twist of the story, Dumbledore could be under the influence of some ancient and evil magic that makes him want to see Hogwarts destroyed, and he convinces Harry that it's the right thing to do. This would be a huge shock as Dumbledore has always been seen as a wise and good figure in the original books.
It could be one of the headmasters or organizers of the Triwizard Tournament who has a hidden agenda. They might be using the tournament for their own gain, perhaps to gain power or wealth, and Harry's actions to destroy it are thwarting their plans.
The Order of the Phoenix members might also back Harry. They've fought against the dark forces before and if they see that the Ministry has become a threat, they'd side with Harry. Dumbledore's old allies in the Order would believe in Harry's judgment and join him in his mission to destroy the corrupt Ministry.
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.
The main antagonist could be an ancient evil spirit that has been awakened by the mixing of ninja and wizarding magic. This spirit is angry at the intrusion and starts to possess students and teachers at Hogwarts. The ninja and Harry Potter must figure out how to exorcise the spirit before it's too late and Hogwarts is completely taken over.
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.
A dark wizard or witch from outside Hogwarts could be the main antagonist. They might target Harry specifically because of his reputation as the 'Chosen One' and use their influence to get him expelled from Hogwarts so that he is more vulnerable and easier to attack.
The main antagonist could be an old, corrupt house - elf. This house - elf was mistreated in the past at Hogwarts and has now turned to the dark side. It wants to bring chaos to Hogwarts and sees Lady Hogwarts as the key. It uses its knowledge of the school's hidden passages and secrets to plot against her. Harry Potter, being the defender of the underdog, tries to reason with the house - elf at first but when that fails, he has to use magic to stop it from harming Lady Hogwarts.
Definitely Hermione. She would not only help with the magical aspects but also with the organization of the cooking process. The Weasley twins might also show up to add some fun and chaos. They could use their inventiveness to create some really unique cooking tools or ingredients. And of course, Hagrid could be involved. He might provide some really interesting and hard - to - find ingredients from his own little garden or from his trips into the Forbidden Forest.
Perhaps there's a member of the royal court who is secretly envious of Harry's power and tries to undermine him. This person could be plotting against Harry and trying to make the queen believe that he is a danger rather than an asset. They might spread false rumors or create false evidence to get rid of Harry.
Perhaps it's an old enemy from Harry's past, like Draco Malfoy who has turned to the dark side completely. He blames Harry for all his misfortunes and decides to cage him without his glasses as a form of cruel punishment.