Another option for the main antagonist could be a group of ice ninjas who believe that their ice - based powers make them superior. They want to subjugate all other ninja villages and make them bow to their icy rule. Naruto and his allies would have to fight against this group's tyrannical ideas and their powerful ice - jutsu.
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.
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.
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.
It could be a new character who has ice - based powers and is out to take over the ninja world. This antagonist might see Naruto as a threat to his plans because of Naruto's growing power and popularity.
The main antagonist could be a powerful Ice Age beast that has been corrupted by dark chakra. This beast might see Naruto and his friends as invaders in its territory and attacks them without mercy. It could have abilities like freezing its opponents with a single breath or being able to move at incredible speeds on the icy terrain.
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.
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.
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 be a new character, an ice ninja named Hakuji. He has a tragic past, his family was killed by a rogue ninja group and he was left alone in the cold northern lands. He comes to Konoha seeking revenge at first but then befriends Naruto and changes his path.