It could be Orochimaru. He might use Naruto's actions of breaking something related to Tsunade (like a seal she was relying on) to his advantage. Orochimaru could manipulate the situation to make Tsunade look weak or to cause chaos in the village.
Another possibility is that the main antagonist could be a new character created specifically for this fanfiction. This character could be a powerful ninja who has a grudge against Tsunade and uses Naruto's accidental breakage (of say, a magical artifact) to start a conflict between Tsunade and Naruto, and then take advantage of the chaos to further their own evil plans.
Perhaps Danzo could be the main antagonist. If Naruto breaks Tsunade's political standing in some way, Danzo would see it as an opportunity to gain more power. He might spread false rumors or use the situation to undermine Tsunade's leadership, all while Naruto is trying to fix what he's done.
Another possibility is that there is no real 'antagonist' in the traditional sense. Instead, it could be the circumstances that act as the 'antagonist'. For example, the differences in their worlds, their duties, and the fact that they come from such different backgrounds could be what causes Erza to break Naruto's heart.
It could be Orochimaru. He might try to disrupt Tsunade's teachings to Naruto because he fears a more powerful Naruto under Tsunade's guidance. Orochimaru could send his minions to attack them during training sessions or steal scrolls that contain the knowledge Tsunade is about to teach Naruto.
Well, it could be about Naruto accidentally breaking something precious of Tsunade's, like a family heirloom. This might lead to a series of events where Tsunade is initially angry but then Naruto makes it up to her in some heroic way, perhaps by going on a difficult mission for her.
Perhaps a new character could be the main antagonist. This new character could be a powerful ninja from another village who has some sort of connection to Konoha. They might be influencing Tsunade's decisions in a negative way, for example, by making deals that Naruto sees as harmful to Konoha. Naruto snaps at Tsunade because he doesn't understand why she is going along with this new character's demands. This new character then becomes the main source of conflict as Naruto tries to expose their true intentions and fix the rift between him and Tsunade.
There might also be other characters involved, like Sakura. Since she was Tsunade's apprentice, she could be part of the story. Maybe she's caught in the middle when Tsunade has to take some action regarding Naruto. Also, Kakashi could be in the fanfic. He has a close relationship with Naruto and might be affected by whatever Tsunade does to Naruto or be involved in the events that lead to Tsunade's actions.
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.
Perhaps in the fanfic, Tsunade breaks Naruto's over - confidence. She might see him getting a bit too cocky and decides to put him in his place. She could do this through a series of tough training sessions or by making him face opponents that are stronger than he initially thought. This would be a way to humble Naruto and help him grow as a ninja.
The pure - blood family that arranged the marriage contract could be the main antagonist. They might have a lot to lose in terms of status, power, or wealth if the contract is broken. So, they would do everything in their power to force Harry to uphold it, even using dark magic or underhanded tactics.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
Maybe a new character could be introduced as the main antagonist. This character could be from a hidden village that has been secretly observing Naruto's growth. They realize his potential as a weapon and plan to steal his power or use him to start a war against other villages.