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.
The antagonist could be a group of anti - ninja forces. They might oppose the values that Naruto represents, like peace and unity among the ninja clans. These forces could have been gathering strength while Naruto was gone and now they see his revival as a chance to start a full - scale war against the ninja villages that follow Naruto's ideals.
Perhaps an ancient evil that was sealed away long ago is awakened when Minato is revived. This entity could be a powerful demon or a dark ninja from the past. It wants to destroy the ninja world and Minato stands in its way. Minato, with his skills and knowledge, has to figure out how to defeat this ancient evil with the help of Naruto and the other Leaf Village ninjas.
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.
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.
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 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.
An unexpected ally could be an old enemy from his past. Maybe a Hollow that he once fought and defeated now sees a greater threat in the situation after Ichigo's revival and decides to team up with him. For example, the Hollow might have information about the new forces that are emerging since Ichigo's return and offers to share that in exchange for some form of protection or mutual benefit.
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.
It could be Team Rocket as always. Since Ash is now a Pikachu, they might see it as an easy target to capture. They could have new gadgets specifically designed to catch Pikachu - Ash, and constantly try to get their hands on him throughout the fanfiction.