One possible antagonist could be an Akatsuki member who has somehow gained knowledge of Naruto's Arthurian reincarnation and wants to use it for their own evil purposes. Maybe they want to steal the power that comes with it.
It could be a rogue ninja who believes that Naruto's newfound Arthurian powers disrupt the natural balance of the ninja world. This ninja might form a group of like - minded individuals and constantly try to undermine Naruto and his allies.
A powerful and ancient demon that was once sealed away by King Arthur could be reawakened in the Naruto world. Recognizing Naruto as Arthur's reincarnation, it would seek to destroy him before he can become strong enough to seal it away again, thus becoming a major antagonist in the fanfiction.
Definitely Voldemort. However, there could also be some Slytherin students who are jealous of Naruto's power and popularity. They might try to undermine him and side with the Dark Lord. And in the background, there could be some dark creatures that Naruto has to face, like dementors or basilisks, which are influenced by the Dark Arts and Voldemort's presence.
It could be a group of rogue ninjas who want to capture the dragons for their power. They might be led by a powerful and greedy ninja who sees the dragons as a means to gain ultimate power over the ninja world.
Another possible antagonist could be a character from Naruto's own world who has also been brought to this new world and has been corrupted by the power of the Witch King. This character might envy Naruto's new - found power as the Witch King and wants to take it for himself. So, he teams up with the dark forces in this new world to oppose Naruto, creating a lot of challenges for Naruto to overcome.
The White Walkers could be major antagonists. Naruto would have to use his skills to combat their undead hordes. Their magic is very different from the chakra - based abilities Naruto has, so it would be a great challenge.
Maybe the Hidden Leaf Village elders could turn into antagonists. They might fear Naruto's new power as the demon king and try to undermine him. They could form alliances with other villages to try and take down Naruto. Also, there could be some new, unknown powerful forces that emerge because of Naruto's transformation. These could be ancient evils that were once sealed away but are now awakened by Naruto's new demonic presence.
It could be a group of rogue ninjas who want to use the power of the Monster King for their own evil purposes. They might try to capture the Monster King and force it to do their bidding, which would put them in direct conflict with Naruto.
There could be a new dark sorcerer who emerges. This person might be jealous of Merlin's position as king and a powerful sorcerer. They would try to bring chaos to the kingdom and usurp Merlin. Also, the neighboring kingdoms that are envious of Merlin's prosperous kingdom could send spies or even start wars, acting as antagonists.
The Sound Ninjas could be great antagonists. They could use their own unique parkour - like skills to try and stop Naruto. They might be chasing him through the forest, and Naruto has to use his parkour skills to outmaneuver them.
Maybe some corrupt officials within the prison system itself. They oppose Naruto's more humane and reform - focused approach to prison management. They might try to undermine him at every turn, for example, by cutting off resources or spreading false rumors about him among the prisoners.
Maybe a group of dark - type Pokémon that have banded together and are trying to overthrow Arceus. They view Naruto as an easy target at first, but quickly realize he is a formidable opponent with his ninja skills and new Pokémon - related powers. They would constantly try to undermine Naruto and his efforts to protect the Pokémon world.