Perhaps the main antagonist is a new character introduced in the fanfiction. This character could be from a different village and is trying to find out the secrets of Naruto's hidden genius in order to use it against Konoha. They might have their own motives for wanting to control or destroy Naruto's potential.
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.
Another option is that the main antagonist is an old enemy who has somehow discovered that Naruto is hiding his genius. They then become determined to stop Naruto from fully realizing his potential. This could be someone who has a vendetta against Naruto or Konoha and sees his hidden genius as a threat to their plans.
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.
Sasuke could be one of the people who help him. Despite their past differences, they have a deep bond. Sasuke has his own skills and knowledge that could be useful in Naruto's situation, like his Sharingan abilities which could help in detecting enemies while Naruto is hiding his face.
It could be a new character. Maybe a powerful ninja from a far - off land who has heard rumors of Naruto's strength and is determined to draw it out and defeat him to prove his own superiority. This antagonist might be secretly spying on Naruto and trying to find ways to force him to show his true power.
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.
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.
Perhaps Kaguya Otsutsuki is resurrected again, but this time with a stronger will and more dangerous plans. Since Naruto was the one who played a major role in defeating her before, she targets him first in this fanfiction.
It could be an evil wind - based ninja who has been misusing the power of the wind. Naruto gives his body to the wind to combat this antagonist directly on its own element's territory.
Perhaps an unknown and powerful dark ninja. This ninja could be from a long - forgotten clan with dark powers. They might have a vendetta against the Leaf Village and see Naruto as the key to their revenge. So they use their dark arts to make Naruto lose his sanity and turn him against his own village.
Maybe a new, powerful and evil ninja from another land. This ninja could have come to the village and caused a lot of chaos, making Naruto believe that no one in his village was capable of protecting themselves or him. So he blames everyone for their weakness and starts to hate them all, with this new ninja being the root of the problem.
It could be a corrupt ninja from another village who wants to get rid of Naruto as he sees him as a threat to his own plans. This ninja could have framed Naruto to send him to prison.