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.
Maybe it's a new character, a powerful wizard who has a vendetta against Harry for some reason. This wizard could be jealous of Harry's fame or power, and Harry has to hide behind a mask to outwit this antagonist who is constantly trying to expose and harm him.
Perhaps a group of rogue Hogwarts students who have fallen under the influence of dark magic. They want to use Harry for some dark ritual, and Snape becomes an unexpected shield for Harry. Snape has to deal with the internal problem of the school while also protecting Harry from these students.
Perhaps in this fanfiction, Harry hides behind a mask because he wants to experience life in a different way. He might be tired of all the expectations that come with being Harry Potter. By wearing the mask, he can interact with people without them knowing who he really is, and he could discover new things about the wizarding society and its inhabitants. He could also be using the mask as a form of self - discovery, to find out who he is when he's not 'Harry Potter'.
Well, one possibility could be that Naruto is hiding his true identity or emotions. Maybe he's on a secret mission and the mask helps him blend in or deceive the enemy. He could be using it as a form of protection, not just physically but also emotionally.
He could be hiding from Orochimaru. Orochimaru has always been interested in Naruto's power. Maybe Naruto has discovered a new power that Orochimaru wants even more desperately, so he hides behind a mask to avoid Orochimaru's detection.
It could be Sasuke. He might be wearing a mask for some mysterious reason, like hiding his new powers or a new allegiance. Maybe he's on a secret mission that even Naruto doesn't know about at first.
It could be someone who is jealous of Naruto's hidden genius. Maybe a fellow ninja who senses that Naruto is hiding something and wants to expose him out of envy. This person might try to undermine Naruto at every turn.
A new dragon - hunter could be the main antagonist. He might be extremely skilled and ruthless, and Hiccup hides his strength to better understand this new threat and find a way to defeat him without putting his village in immediate danger.
It could be a new dark wizard who rises to power in the wizarding world. This wizard blames Harry for leaving and believes that his departure has made the wizarding world weaker. So the new dark wizard sends his minions to the Muggle world to force Harry to return to the wizarding world and face him, or else he will start causing chaos among Muggles which Harry has come to care about.
The Joker could be a likely antagonist. He might be causing chaos in Gotham, and Robin has to hide in Batman's cape to avoid being spotted by him. The Joker's unpredictable nature would add tension to the story.
It could be someone who also wants the power of the mask. Maybe a rogue ninja who has been researching the mask for a long time and sees Naruto as an obstacle to obtaining it.