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.
The consequences could be quite complex. On one hand, Naruto may be angry at Sasuke for breaking his mask. But on the other hand, it could also be a turning point. It might force Naruto to confront the parts of himself that he was hiding, and in a way, it could bring them closer in the long run as they understand each other better. However, initially, there would likely be a lot of tension and conflict.
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.
The plot might involve Naruto breaking the mask and finding out that Sasuke has been manipulating events behind the scenes to his own advantage. For example, Sasuke could have been the one causing the rifts between the different ninja clans in order to gain power. Once Naruto breaks the mask, he has to convince the other characters of Sasuke's wrongdoings. This could lead to a big divide in the ninja community, with some believing Naruto and others still blindly following Sasuke.
It changes the power dynamic. Naruto, by breaking the mask, becomes the one in control of the truth. Sasuke, who may have been seen as a cool and mysterious character before, is now at a disadvantage. This can lead to a shift in alliances as other characters start to side with Naruto.
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.
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.
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.
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 also be a physical mask breaking in a more action - packed sense. For example, if there's a battle where a masked enemy is fighting Naruto and his allies, and the mask breaks, it could reveal a shocking identity. This could add an element of surprise and suspense to the fanfic, making it more engaging for the readers.
In the fanfiction, Naruto's emotionless mask might make Sasuke feel like he's lost his rival. Naruto's lack of emotional response could cause Sasuke to question if Naruto still cares about their bond. Sasuke might start to distance himself, thinking that Naruto has changed too much. However, as the story progresses, Sasuke could realize that the mask is just a facade and they could start to repair their relationship.