There might be a corrupt Quidditch official who has a grudge against Harry or the team he joins. This official could try to make unfair calls during the matches to make Harry's team lose.
Perhaps an opposing team's star player. This player might be threatened by Harry's reputation and skills. So, he tries to intimidate Harry before the big matches and plays dirty during the games.
It could be a jealous teammate who feels that Harry is getting all the attention and glory. This teammate might try to sabotage Harry during practices or matches.
It could be a new player on the team who is jealous of Harry's skills and popularity. This new player might spread lies about Harry, turning the other team members against him. So the new player would be the main antagonist in this scenario.
Well, the story might start with Harry getting an unexpected invitation to join a professional Quidditch team. He has to quickly adapt to the more competitive and cut - throat environment. His old Hogwarts friends might be jealous or proud. As he progresses in the team, he could be involved in a big tournament where the fate of the team depends on his performance.
Ron and Hermione could still be important characters. Ron might be super excited for Harry and offer some support and advice. Hermione could be the one making sure Harry doesn't neglect his studies while being on the team.
In this fanfiction, Harry might face intense training sessions. He could struggle at first to keep up with the more experienced players on the team. Maybe he'll have a rival on the team who doubts his abilities but he'll prove them wrong with his natural talent.
Perhaps an old enemy of one of the members of Team 9 shows up. Since Naruto has joined the team, they might see it as an opportunity to get revenge. This antagonist could be someone who was once defeated by the member's family and now wants to prove themselves. They would be very cunning and use their knowledge of the team member's past to try and divide the team.
Dumbledore could be seen as the main antagonist from Harry's new perspective on the dark side. Dumbledore represents all that is good in the wizarding world, and if Harry has turned to the dark, Dumbledore would be the one trying to stop him, to bring him back to the light. So in this context, Dumbledore becomes the opposition that Harry has to face.
Orochimaru could still be the main antagonist. Even if Naruto joins Team Hebi, Orochimaru's motives are often self - serving and evil. He might be using the team for his own experiments or to gain power, and Naruto would oppose that. Naruto's presence in the team could disrupt Orochimaru's plans, leading to a conflict between them.
There might be a supernatural or magical antagonist, even in a military setting. This could be a dark wizard or a creature that has infiltrated the military for its own malicious purposes. Harry, with his experience in the magical world, would be the key to dealing with this threat, but he has to convince the non - magical military personnel of the danger first.
It could be Voldemort. Imagine if he somehow found a way to cross over into the world of the Fellowship. He would be a powerful and menacing antagonist, using his dark magic to oppose both Harry and the Fellowship.
It could be Voldemort. Even if it's a fanfiction where Harry joins Tom, Voldemort's future self might still be a threat and oppose Harry's attempts to change Tom.