Madara Uchiha could also be a main antagonist. He has grand plans for the world and Naruto becoming a tailed beast could disrupt his plans. He may see Naruto as a new threat that needs to be eliminated or controlled. Madara might try to use his Sharingan powers to control Naruto's tailed - beast - like state and turn him against the Leaf Village.
It could be Orochimaru. He has always been interested in power and the secrets of the tailed beasts. He might see Naruto's transformation into a tailed beast as an opportunity to study and use that power for his own evil purposes. He could try to manipulate Naruto in his new state, perhaps by using genjutsu or other underhanded tactics.
In this fanfiction, the plot might involve Naruto being experimented on by some dark force. This experiment causes him to transform into a tailed beast in a way that's different from his normal jinchuriki forms. As he changes, he starts to view the world from a more primal perspective. He might even break away from the Hidden Leaf Village and the story could be about his journey to regain his sanity and his place among his friends and the village.
It could be a rogue ninja from a hidden village who has a vendetta against the Green Beast. Maybe this antagonist blames the Green Beast for something that happened in the past.
Well, Naruto himself might still be a main character as he has a strong connection to the tailed beasts in the original series. He could be the one trying to understand and deal with the 10 - tailed beast in new ways.
It could be an original character. For instance, a young girl from a small, forgotten ninja clan who has a natural affinity for the tailed beasts. Maybe she has a special kekkei genkai that allows her to sense their emotions from afar, and she embarks on a journey to conquer the tailed beasts to prove her clan's worth.
It could be a rogue ninja who wants to capture the ten - tailed wolf for its power. He might be someone who was once shunned by the ninja village and now seeks revenge by obtaining this ultimate power source.
It could be a new, powerful group that has emerged with the sole purpose of taking the tailed beasts from the reader. They might have some unique jutsu or technology that makes them a formidable foe. Maybe they are a group of rogue ninja from different villages who banded together out of envy for the reader's power.
He could be experimented on by rogue ninja scientists. These scientists might have been researching ways to create super - powerful beings and see Naruto as the perfect test subject. They use some forbidden jutsu or strange technology to try and force the creation of an 11 - tailed form within Naruto.
Perhaps a rogue werewolf pack. They see Stiles as a threat because of his nine - tailed fox powers. They believe that his existence could disrupt the balance of power in the supernatural world. So, they constantly try to hunt him down and drive him out of the town. They might also target his friends to get to him.
The main antagonist could be the sorcerer who cast the spell on Belle to turn her into a beast. This sorcerer might have his own motives, like wanting to take over the village or the castle where Belle lives. He could be constantly trying to prevent Belle from finding a way to break the curse.
It could be an envious witch. She was once in love with the prince (now the Beast) but he rejected her. So she cursed him and now she wants to make sure he never finds true love with the Dark Beauty.