Perhaps there is no traditional antagonist. Instead, Naruto's own insecurities could be the 'antagonist'. He's so used to wearing the mask of the happy - go - lucky guy that his inner self is resistant to being exposed. So, in a way, his own fear of change and vulnerability is what Sakura has to fight against when she tries to break his mask.
Another possibility is that it could be a new character created specifically for the fanfiction. A character who wears a mask for some secretive purpose, and when the mask breaks, it sets off a chain of events that are unique to the fan - created storyline.
Sakura could develop more empathy. By breaking Naruto's mask, she sees a side of him she didn't before. This might make her more understanding and less judgmental. For instance, if Naruto has been hiding his pain, Sakura's realization could soften her towards him.
Maybe it's a jealous rival of Naruto who wants Hinata for himself. He manipulates situations in such a way that Hinata ends up hurting Naruto. For example, he could stage events to make Hinata think Naruto has betrayed her trust.
Ino could be seen as the antagonist in this fanfiction. Since she's the one breaking Naruto's heart, she's causing the main conflict. Her actions are directly opposed to Naruto's happiness, which is a typical trait of an antagonist.
Well, it could be that Sakura discovers Naruto has been hiding some big secret behind his usual go - lucky mask. Maybe she finds out he's been suffering alone about some dark past related to his parents or the Nine - Tails. And in a moment of intense emotion, she breaks his mask, which is more of a metaphorical mask representing his false bravado. This could lead to a deeper bond between them as Naruto finally opens up.
It could be Sasuke. He often has a cold exterior, like a mask, and if in a fanfic his mask breaks, it could mean he finally shows his true feelings towards Naruto, whether it's friendship, rivalry, or something more complex.
In this case, Sakura could be seen as the antagonist, at least in that particular moment when Naruto tells her off. Since Naruto is the one doing the telling - off, it's because of something Sakura has done or said that has pushed him to that point.
One possible plot could be that Sakura discovers Naruto has been hiding his true emotions or identity behind a metaphorical mask. She might break it by having a deep and honest conversation with him, forcing him to confront his real self. For example, Naruto has always shown a tough exterior but Sakura realizes he's actually very vulnerable inside.
It could be out of jealousy. Sasuke might see Naruto as always hiding his true emotions and having this perfect image that others admire. By breaking the mask, he wants to expose Naruto's flaws and make himself feel superior. Or perhaps it's a way for Sasuke to assert his dominance over Naruto, to show that he can break through Naruto's defenses.
It could be Voldemort. Even if he was defeated in the original story, in this fanfiction he might have some sort of lingering influence or perhaps a follower who is continuing his dark work. Harry hides behind the mask to avoid being detected by this new threat related to Voldemort.