The Ministry of Magic could become the main antagonist. They might suspect Harry of being involved in some dark magic because of his isolation. They could start to hunt him down, thinking he's a danger to the wizarding community. This would put Harry in a difficult position as he tries to prove his innocence while being alone.
Voldemort could still be the main antagonist. Even though Harry is alone, Voldemort would see him as a threat and continue to send his Death Eaters after him. Harry would have to face these attacks all by himself, which would make the story very intense.
Voldemort could still be a great antagonist even in a story where Harry battles alone. His power and the threat he poses are well - established in the Harry Potter universe.
It could be a new dark wizard who is more powerful and cunning than Voldemort. This wizard might have been in hiding all this time, biding his time to take over the wizarding world. With Harry being alone, he would pose a great threat as Harry doesn't have his friends to support him in this fight.
A great antagonist could be a corrupted teacher. For instance, a teacher who is secretly working for Voldemort but has been hiding in plain sight at Hogwarts for years. They could manipulate the Triwizard Tournament to their own ends, causing all sorts of trouble for Harry and his friends.
The main antagonist could be an internal one - Harry's own guilt. It's like a dark shadow that follows him everywhere. He wants to be alone to escape from it, but it keeps haunting him. It makes him question his every move and decision, and it's this internal struggle that could be seen as the main 'antagonist' in the fanfiction as it constantly opposes his desire for peace and solitude.
One possible storyline could be Harry feeling isolated and betrayed by his friends at Hogwarts. He might decide to go off on his own adventures, exploring parts of the wizarding world that are less known, like some hidden magical forests or ancient ruins.
It could be a new and powerful dark ninja who thrives on isolation and wants to keep Naruto alone. He might see Naruto's solitude as a weakness to be exploited.
Hermione could be a main character. She might be one of the few who still believes in Harry and tries to reach out to him despite his isolation. Another could be Dobby. Dobby, being the kind-hearted house - elf, could bring Harry food and supplies while he's in isolation.
One possible plot could be that after the events of the fourth year, Harry discovers that some of his friends and allies are starting to doubt him because of the return of Voldemort. This makes him feel isolated. He might then start to focus more on self - study of magic, exploring hidden parts of Hogwarts on his own.
A group of bandits might be the main antagonists. They see Percy as someone they can easily rob or enslave since he is all alone. Percy would then have to use his skills to outwit them.
There's a possibility that the main antagonist is an internal force within Naruto himself. His desire to be alone could be a manifestation of some dark part of his psyche. This 'inner antagonist' could be trying to make Naruto cut himself off from his loved ones completely, leading him down a path of self - destruction. And Naruto has to fight this internal battle while also dealing with any external threats that come his way during his isolation.