Another possibility is that the antagonist is a powerful and ancient magical being that has its own agenda. This being doesn't want Harry to reach his full potential as Dumbledore hopes because it fears that a powerful Harry could disrupt the balance it has been maintaining in the magical world for centuries. So it tries to create obstacles for Dumbledore's efforts to help Harry.
One possible plot could be that Dumbledore wants Harry to take on a dangerous but crucial mission earlier than in the original story. He might be desperate because he sees a dark threat approaching faster and believes only Harry has the unique power to stop it.
Well, in this kind of fanfiction, Dumbledore himself would be the main antagonist since the title implies that he is the one 'breaking' Harry. He would be acting against Harry's well - being and goals.
It could be Voldemort. He might have found a way to manipulate Harry into attacking Dumbledore's office. By doing this, he not only causes chaos in Hogwarts but also tries to drive a wedge between Harry and Dumbledore.
Dumbledore himself could be seen as the main antagonist in this scenario. Since Harry is set on taking him down, Dumbledore's actions, whether they are truly malicious or just misinterpreted, would be the main obstacle for Harry to overcome.
Another possible antagonist could be a group within the wizarding world who doesn't want Dumbledore's plans to be exposed. They might have been benefiting from Dumbledore's secrecy or they believe that his plans, although seemingly unethical to Harry, were necessary for the greater good. They would then try to stop Harry from revealing the truth.
It could be Voldemort. If this story is set during the time when Voldemort was still a threat, he might be trying to stop Harry from getting home. Maybe he has a plan that involves keeping Harry away from the safety of his home so that he can be more easily attacked or manipulated.
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.
The antagonist could be a new, self - proclaimed ninja god who wants to rule over all the villages. He sees Naruto as the only threat to his power, so he used a powerful spell to take away Naruto's godlike abilities. Konoha wants Naruto back to stop this tyrant. This antagonist might have an army of brainwashed ninja at his command.
It could be an old rival from Percy's past, someone who has always been jealous of Percy's abilities and has finally found a way to cause him great harm. This person would be a perfect antagonist as Percy would have a strong motivation to get revenge on them.
There could be various reasons. Maybe the person Dumbledore wants for Harry doesn't have a good personality or they have different goals and values. Or it could be that Harry simply doesn't feel a connection with that person.