A music producer could be the main antagonist. If Beca Mitchell refuses to follow the producer's unethical demands, like changing her style to something she doesn't believe in, the producer might see her as a liability. So, in order to protect their own reputation and power in the industry, they decide that Beca Mitchell must die, at least in the context of the fanfic.
Perhaps some of the other ninja villages see Naruto as a potential danger in the future, so they manage to infiltrate Konoha and make it seem like Naruto has committed a great crime, resulting in him being left in the forest. In this case, the main antagonist would be the ninja from the other villages.
It could be a new, powerful ninja from a hidden village that has a different view on power. This ninja might believe that Naruto's power should be harnessed for their own village's supremacy. So, they try to turn Naruto into a weapon against his will. They would be a formidable opponent, with skills and abilities that Naruto has not encountered before.
It depends on the specific story. Different stories have different antagonists. Sometimes it could be a villain with evil intentions, and other times it might be a character with opposing goals to the protagonist.
An antagonist is basically a character or a force that opposes the main character or protagonist. They create conflicts and obstacles for the hero to overcome.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.