It could be Orochimaru. He's always interested in powerful artifacts, and the mask Naruto gets would be no exception. Orochimaru would likely try to steal the mask from Naruto to add to his collection of powerful items and use its power for his own dark experiments.
Another possible antagonist could be a new character created specifically for the fanfiction. This character could be a rogue ninja who has a personal vendetta against Naruto's village. He hears about the mask Naruto has obtained and sees it as a way to get revenge. He might be more powerful than Naruto at first, but Naruto's growth with the help of the mask would eventually turn the tables.
Maybe an old enemy who was thought to be defeated but has somehow returned stronger. Orochimaru could be a possibility. He was always obsessed with power and immortality. If Naruto is revived, Orochimaru might see it as an opportunity to study Naruto's new state of being (since he was revived) and use that knowledge for his own dark purposes.
Sasuke could potentially be an antagonist. With Naruto having the Rinnegan, Sasuke might see him as a threat to his own power and goals. This could lead to a new rivalry between them, different from their previous one.
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.
It could be Sasuke. I know they are friends in the original, but in this fanfiction, Sasuke might turn against Naruto for some reason. Maybe he becomes jealous of Naruto's newfound power or status.
A new and powerful ninja from a far - off land could be the main antagonist. This ninja has heard of Benihime's power and wants it for himself. He sees Naruto as an easy target at first but soon realizes that Naruto is not going to give up Benihime without a fight. This antagonist could have unique powers that are a challenge for Naruto to overcome while protecting Benihime.
It could be Orochimaru. He has always been interested in power and Naruto's unique abilities. With Naruto sealed away, Orochimaru might see it as a chance to steal whatever power Naruto had or use his absence to further his own evil plans in the ninja world.
It could be someone who is jealous of Naruto getting the monkey contract. Maybe a rogue ninja who wants the power for himself.
Well, if Naruto got a hollow mask in fanfiction, he might gain new powers. The hollow mask could potentially enhance his speed and strength, like how it did for characters in Bleach. He could also use it to intimidate his enemies. Maybe it would give him a new form of chakra or a special jutsu that combines his ninja skills with the power of the hollow mask.
It could be a hidden group within Konoha that was behind Naruto's banishment. They might be afraid of his growing power and influence. This group could work in the shadows to stop Naruto from ever returning to Konoha and regaining his status.
His enemies, too. They might realize that Naruto is not as straightforward as they thought. If he shows a more cunning side by dropping the mask, they would have to adjust their battle strategies. For example, Orochimaru might find Naruto a more formidable opponent than he initially believed.