Another possibility is a new character entirely. This new character could be from a far - off land and wearing a mask as part of their culture or because they are on the run from something in their homeland. They come to the Hidden Leaf Village and get involved with Naruto's adventures, and as the story progresses, the mystery of the mask and their true identity unfolds.
Kakashi is a likely candidate. He already has a history with wearing a mask. In a fanfic, there could be more secrets behind his mask that are yet to be revealed. For example, his mask could be hiding some unique kekkei genkai that he's been keeping under wraps. Or perhaps he's wearing a different kind of mask, not just the physical one, but an emotional mask to hide his true feelings about certain events in the past.
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.
Sasuke could be a great candidate. He has always been a bit of an enigma, with his brooding nature. He could be hiding a new power or a different side of his personality behind a mask, either self - imposed or as a result of some outside influence. Maybe he's trying to hide his true intentions from Naruto and the others as he pursues his own goals.
It might also be a new character created specifically for the fanfiction. This character could be someone who has infiltrated the Hidden Leaf Village, living behind a mask of false loyalty while having their own secret motives and goals.
Astrid is also a possibility. She's a strong - willed character, but perhaps there's more to her than what we usually see. She could be hiding her softer emotions or insecurities behind a tough - girl mask. For example, she might be secretly worried about her relationship with Hiccup or her place in the tribe.
One possibility is that the story could focus on a character who has been living a double life. They are hiding their true self behind a mask, perhaps for protection or because they have a secret mission. In the Naruto universe, it could be a ninja who is secretly working against a powerful enemy within their own village. They use the mask to conceal their identity and actions from those around them.
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.
Kakashi could be another option. He is a highly experienced and intelligent ninja. If he came across the skull mask, he would approach it with caution. He might study it first to understand its origin and power before deciding whether or not to use it. His calm and collected nature would be a great contrast to the potentially wild power of the mask.
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.
The Riddler could be added. He could be constantly trying to figure out what lies behind Batman's mask, both literally and metaphorically. His mind - games could force Batman to confront things about himself that he usually keeps hidden.
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.
Another theme could be isolation. When a character lives behind a mask, they are often alone in their deception. This fanfiction might show how that character struggles with loneliness, like not being able to truly connect with others because of the false facade they present.