A powerful organization that wants to use the power of the Fox King for world domination could be the main antagonist. They see Naruto as an obstacle because of his connection to the Fox King. JM, being associated with Naruto, also becomes a target. This organization might have their own set of powerful ninjas and dark jutsu, and they stop at nothing to achieve their goal of harnessing the Fox King's power.
A powerful rogue Shinigami could be the main antagonist. This Shinigami might be jealous of the Shinigami King's power and wants to overthrow him. He sees Naruto as a threat because of his connection to the king and tries to stop Naruto from fulfilling any tasks or missions related to the king. He could also be causing chaos in the living world by releasing dangerous spirits or curses.
Orochimaru would be a great main antagonist. He's always been interested in power and the secrets of the body. Naruto's half - demon fox nature would be a prime target for his experiments. He could try to extract the power of the demon fox from Naruto, or use Naruto as a way to create his own brand of super - soldiers with demon - like powers.
One plot idea could be Naruto exploring his connection with the fox spirit in a more spiritual and ancient way. Maybe he discovers hidden powers within himself related to the Fox King that are not just about combat but also about protecting the natural balance. Another idea is that Naruto meets a character named JM in the fanfic who has a special ability to communicate with the Fox King on a different level, and they team up to face a new threat to the ninja world.
Another possibility is that there is a group of anti - jinchuriki people who see Naruto as a threat because of his connection to the giant fox. They might try to kill Naruto to get rid of the 'giant fox problem'. Their leader could be the main antagonist, someone who has a deep - seated hatred for anything related to the tailed beasts.
It could be a powerful ninja who wants to capture the fox brother for his own purposes. Maybe he believes that the fox brother's powers can be used to take over the ninja world. This antagonist might be from a rival village or a secret organization within Naruto's own village.
It could be a rogue fox - clan member who wants to use the power of the Nine - Tailed Fox inside Naruto for his own evil purposes. He might try to manipulate Naruto or directly attack the Fox Clan to get what he wants.
The main antagonist could be one of the village elders. They might see Naruto's fox - summoning abilities as too dangerous and banish him out of fear. Throughout the fanfiction, they could try to prevent Naruto from coming back to the village, fearing that he'll seek revenge or disrupt the power balance they've established.
Maybe the main antagonist is an ancient spirit that has been awakened by Naruto's experiments with transformation. The spirit doesn't like Naruto's interference with nature's balance and decides to punish him by making him turn into a fox with the strange 'lemon' aspect. This spirit then tries to keep Naruto in this form permanently and drive him mad, so that Naruto can no longer be a threat to the spirit's plans of ruling over the ninja world.
The Organization in the Claymore world could be the main antagonist. They already have a lot of secrets and do some morally ambiguous things. If Naruto shows up and starts changing the status quo, like teaching Claymore warriors to be more independent, the Organization might see him as a threat and try to eliminate him.
It could be a rogue ninja who was once a student of Orochimaru and has now developed his own dangerous jutsu. He wants to take over the ninja world and believes the old ways are holding everyone back.