The Ministry of Magic could be the main antagonist. They might be the ones who spread lies or misinformation about Harry, causing everyone to abandon him. They could be afraid of Harry's power or have some ulterior motives to get rid of him. They could use their influence and authority to turn the public against Harry, making his life a living hell as he tries to survive on his own.
In some cases, it could be an ancient and evil magical being that has been awakened. This being has the ability to manipulate people's minds and convinces everyone that Harry is evil. It wants to isolate Harry before attacking him directly for some unknown but dark reason.
The Ministry of Magic itself could be the main antagonist. They might be so afraid of Harry's growing power and influence that they create false evidence to make everyone believe he has betrayed the wizarding world. They try to control the situation by turning everyone against Harry.
Well, it could start with Harry being wrongly accused of something major, which leads to everyone abandoning him. He then embarks on a solitary journey to find the truth and clear his name. During this time, he might encounter old enemies who see his vulnerability and try to take advantage of it. However, Harry's determination and the few skills he has managed to retain despite the abandonment help him fight back. He also has internal battles with feelings of loneliness, betrayal, and self - doubt, but ultimately he rises above it all to show that he is still a force to be reckoned with.
It could be a rival trainer who tricks Pikachu into leaving Ash. This trainer might be jealous of Ash's bond with Pikachu and wants to break it for his own gain.
It could be someone from the village who doesn't want Naruto to discover the truth about his parents. Maybe they fear that if Naruto finds out certain secrets, it will disrupt the power balance in the village.
Maybe the main antagonist is the entire Iwa council. They might be the ones who order Naruto's abandonment because they fear his potential influence on Iwa. They would do everything in their power to keep him from uncovering their plans or causing unrest among the villagers.
It could be Danzo. He's always been a bit of a shadowy figure in the Naruto series. Maybe he sees Naruto as a threat to his power and influence in the village and manipulates the situation so that Team 7 has to abandon him. He would be a great main antagonist as he has the resources and the motives to cause such a rift in Team 7.
The Akatsuki could be the main antagonist. They might see Hanzo as an easy target after Naruto abandons him and try to capture him for their own purposes, like using his knowledge of the ninja world or his unique abilities.
It could be the elders of his clan. They are often the ones who make decisions in the clan, and if they see Naruto as a threat for whatever reason, they could be the main antagonists. They might be afraid of Naruto's growing power or his different way of thinking that could disrupt the traditional ways of the clan.
Maybe it's a new character created specifically for the fanfiction. This character could be a powerful sorcerer who has the ability to manipulate people's minds. He targets Goku out of jealousy for his power and popularity. By using his mind - control powers, he makes everyone around Goku believe that he's a traitor, thus making himself the main antagonist.