There could be a group of older Gryffindor students who have different ideals from Harry. They think Harry is too reckless or that his actions are putting Gryffindor in a bad light. They might see themselves as the protectors of the Gryffindor name and so they attack Harry to try and stop him from what they see as his wrongdoings.
Maybe it's a new Gryffindor student who has been influenced by the wrong people. This new student doesn't really know Harry well and is easily convinced by some anti - Harry propaganda within the school. So they become the main antagonist in the story and lead the attack on 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.
Well, it could be that there's a misunderstanding. Maybe some Gryffindor students think Harry has done something wrong, like he's been framed for a crime in Hogwarts. So they turn against him in anger and attack him. But later they find out the truth and feel really guilty.
A dark force that has the ability to influence the minds of Gryffindor members. This could be a powerful and ancient magic or a new form of dark magic. It turns the house against Harry without them fully realizing it, making the collective Gryffindor house, under this influence, seem like the antagonist.
It could be a new dark wizard who is secretly operating within Hogwarts. He might be trying to steal powerful magical artifacts and Harry and Luna get in his way.
There could be a group of magical creatures who see the snake as an abomination or a threat to their kind. They would try to destroy the snake and Harry would have to protect it. These creatures could be from a part of the Forbidden Forest that has not been explored much in the original books and they have their own set of rules and magic which makes them a formidable antagonist.
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.
Surprisingly, it could be Bellatrix herself. Despite being enslaved, her mind could be so twisted that she constantly tries to break free from Harry's control and undermine his actions. She might try to secretly communicate with Voldemort or other Death Eaters, making her an internal and dangerous antagonist for Harry.
Maybe it's a group within the Ministry of Magic who oppose Harry's influence on Teddy. They might see Harry as trying to groom Teddy to be his successor in some way and want to stop them from leaving together to prevent this.
Perhaps an ancient evil that Gryffindor once sealed away is starting to break free. This entity sees the descendant as a threat because they have the potential to reseal it. So, it tries to corrupt the descendant or stop them in any way possible.
It could be a power - hungry Slytherin who doesn't like that Regulus is showing an interest in Gryffindor values. This Slytherin might see it as a betrayal to their house and try to stop Regulus by any means necessary, even if it means using dark magic.