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.
The main antagonist could also be an outside force, like a Quidditch coach from another team who wants to weaken Harry's team. He bribes or blackmails some members of Harry's team to betray him. This way, the coach from the other team is the real driving force behind the betrayal, even though it's the teammates who actually carry it out.
Perhaps in the fanfiction, the Quidditch team gets influenced by a dark wizard who has a grudge against Harry. The wizard uses a spell to make the team members believe Harry is a threat to them. So they start to betray him by not passing the Quaffle to him during the game, or even colliding with him on purpose during a fly - around. This would lead to Harry feeling extremely isolated and confused as he thought his team was like family.
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.
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.
Perhaps an old enemy who has infiltrated Gryffindor in disguise. They use their influence to make the Gryffindors believe that Harry is a danger. They could be a Death Eater or a follower of Voldemort who has found a way to get into Gryffindor and manipulate the students' minds, causing them to betray Harry.
If Harry's parents are the ones who betrayed him, they could be considered the main antagonists in a sense. They created this false image of themselves as loving parents when in fact they had ulterior motives. Another possibility could be a character who was in on the parents' plan and now is trying to cover it up or continue the malicious work they started. Maybe it's an old acquaintance of his parents from their days at Hogwarts who is now out to stop Harry from finding out the truth.
One great fanfiction is 'The Gryffindor Quidditch Revolution'. It focuses on a new player who brings innovative strategies to the team. The story is full of action - the intense Quidditch matches are described vividly, making you feel like you're right there in the stands. The characters are well - developed, and it stays true to the spirit of the Gryffindor team in the original books.
A group of rogue Death Eaters could be the main antagonists. They think that Voldemort has gone soft by teaming up with Harry. So they break away and form their own more extreme and violent group. They start attacking both the wizarding world and anyone associated with Harry and Voldemort's new alliance. Harry and Voldemort must then fight against their former allies who have become a greater threat than ever.
The sense of adventure. In the original books, the Gryffindor Quidditch team was always at the center of exciting action. Fanfictions build on this. They take the team on new adventures, like facing never - before - seen opponents or dealing with magical mishaps during a match. It's like getting to experience more of the wizarding world through the lens of the team we all love.
The Quidditch matches often had some plot - related events. For example, in some matches, there were attempts by the Slytherin team or others to cheat or play unfairly. This added drama and conflict to the story. Also, Harry's skills as a Seeker were crucial in some key moments, like when he caught the Golden Snitch to win important matches for Gryffindor, which in turn affected the overall house standings in the school.
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.