The established nobles who fear losing their power could be antagonists. They might try to plot against the new king to keep their old ways and privileges. Another potential antagonist could be a powerful and jealous rival who believes he should be the king instead.
Potential antagonists could include a secret organization that has been controlling things from the shadows in the 'Unordinary' world. They see the new king as a threat to their hidden influence. Also, there could be a group of fanatics who follow an extreme ideology and think the new king's rule goes against their beliefs. They would stop at nothing to undermine him. Additionally, a former ally who feels betrayed or overlooked by the new king might turn against him and become an antagonist.
A former acquaintance of Beckett who has turned to a life of crime could be an antagonist. They might have some old grudge against Beckett, like Beckett once reported them for something minor but it led to them having a really hard time, so now they want revenge by holding Beckett hostage.
Another antagonist could be a group within the ninja village itself. Maybe they fear the changes that the child of prophecy might bring. They could be the more conservative elements who don't want to see new ideas or powers emerging. They might try to undermine the child's training or spread false rumors about them. For example, they could claim that the child is a danger to the village, turning public opinion against the child.
A great 'A New King Unordinary' fanfic often has unique plot twists and well-developed characters that expand on the original story.
Shang Tsung could be a great antagonist. He might see Sindel's pregnancy as an opportunity to gain power. Maybe he wants to use the unborn child for some dark magic or experiment. His devious nature would make him a formidable foe against a pregnant Sindel.
One idea could be that the new king has to face a hidden conspiracy within the world of 'Unordinary'. Maybe some powerful groups are trying to undermine his rule from the start. Another could be that the new king discovers a long - lost power that changes the balance of power in the 'Unordinary' universe. He then has to decide how to use it for the good of all.
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.
A rival law firm could be the antagonists. They might try to steal Joe's clients or undermine his cases in unethical ways, like spreading false rumors about his legal abilities.
A group of rogue ninjas who have made the island their base could be antagonists. They might see Naruto as a threat to their operations on the island.
A powerful wizard who wants to use the 'lady of darkness' powers for his own evil gains could be an antagonist. He might try to manipulate her into doing his bidding.