Another possible antagonist could be a rogue ninja from Naruto's own village who has turned against it. This ninja could be jealous of Naruto's status as a guardian and the power he wields. So, he teams up with external forces to try and undermine Naruto's guardianship and take over the role himself or just cause chaos in the village.
It could be Orochimaru. He's always been after power and new jutsu. If Naruto is guarding something really valuable, like a new powerful ninja technique, Orochimaru would be a likely antagonist. He'd stop at nothing to get his hands on it.
The Akatsuki could also be the main antagonists. They are a group of powerful and dangerous ninjas. If Naruto is protecting a person who has some sort of connection to the jinchuriki or a powerful chakra source, the Akatsuki would surely be interested. They might send their best members like Itachi or Kisame to try and get past Naruto and take what they want.
It could be Danzo. He was always a bit of a shadowy figure in the Naruto series. In a ninja tribunal fanfiction, he could be working behind the scenes to manipulate the tribunal for his own gains, perhaps trying to get rid of those who oppose his more extreme views on protecting the village.
It could be Orochimaru. He is always after power and Naruto's new silent and perhaps more powerful form could be a threat to his plans, so he would try to stop Naruto.
Perhaps it could be a powerful ice spirit that has been awakened by some dark jutsu. This ice spirit has no concept of good or evil but is causing destruction as it spreads ice everywhere. Naruto and his friends would have to find a way to either seal it or make it understand the harm it's causing. In this case, the ice spirit would be the main antagonist as it opposes the well - being of the villages.
A powerful and corrupt ninja from another village could also be the main antagonist. This ninja might be jealous of Naruto Regbinks' skills or the attention he gets. He could try to undermine Naruto Regbinks at every turn, starting conflicts between villages and trying to make Naruto Regbinks look bad in the eyes of his own people. This would create a lot of political and personal drama within the fanfiction.
It could be a long - lost relative of Orochimaru. This person might have been secretly studying Orochimaru's forbidden jutsu and developed even more terrifying techniques. They could be seeking revenge on the ninja world for ostracizing Orochimaru.
Genma Shiranui is also a fan favorite. With his unique fighting style, involving his senbon needles, he stands out. In fanfiction, he could be involved in some really intense battle scenes, using his speed and precision to take on powerful foes.
Maybe an entire rival village decides to target the prodigy. They see the potential threat that the prodigy poses to their own village's dominance, so they send their best ninjas to take the prodigy down.
It could also be a powerful and jealous ninja from another village. This ninja is envious of Naruto's growing power and popularity. He decides to create a false narrative about Naruto being a threat to all the villages and starts a war against him. Naruto would then have to prove his innocence and stop the war before it destroys the ninja world.
A great plot for 'naruto as guardian ninja fanfiction' could be Naruto becoming the guardian of a group of young orphans who have special ninja abilities. These orphans are being targeted by an evil organization that wants to use their powers for bad. Naruto has to protect them, train them, and at the same time, keep their existence a secret from the world. He would have to deal with the emotional trauma of the orphans and help them grow into strong and kind - hearted ninjas.
It could be a dark sorcerer who wants to steal the power of the moon for his own evil purposes. He sees Mune as the only obstacle in his way, so he tries to undermine Mune at every turn.