It could be an old enemy of Sirius Black, someone from his past like a Death Eater he had a personal vendetta with. This Death Eater could have found a way to influence Sirius's actions and make him kick out the Order, with the aim of destroying the resistance against Voldemort, making the Death Eater the main antagonist.
It could be Bellatrix Lestrange. Since she is a Death Eater and has a personal vendetta against Sirius, she would be a great antagonist. She might try to kill Sirius again and also target Harry to please Voldemort.
It could be that there is no traditional 'antagonist' in the sense. Maybe it's just the situation and the conflict between Harry and Ron's personalities. But if we had to consider one, it could be an outside force that is influencing Ron's actions and making Harry distrust him. For example, a new dark magic object that is affecting Ron's judgment.
One possible plot twist could be that Sirius Black has a hidden motive for kicking out the Order. Maybe he discovers a traitor within the Order and by kicking them all out, he forces the real traitor to expose themselves in order to rejoin. Another could be that Sirius is being influenced by a dark force that he's unaware of, which is making him act out of character. For example, it could be a spell that was cast on him by a Death Eater.
Voldemort himself could be the main antagonist in this scenario. He might offer Hermione something she desperately wants in exchange for her betrayal. Maybe he promises to spare her Muggle - born friends if she gives up the Order's plans and secrets. His powerful and intimidating presence could easily force Hermione into a difficult situation where she feels she has no choice but to betray.
One possible reason could be miscommunication. Maybe Harry does something that Sirius misinterprets as a betrayal or a stupid mistake, leading to Sirius lashing out.
It could be a powerful dark wizard who wants to use Sakura's powers for his own evil purposes. He might be constantly trying to capture her or manipulate her into doing his bidding.
It could be a rogue Death Eater who escaped justice and is now seeking revenge on the Order and anyone related to them. So, Teddy, being Lupin's son, would be a target.
Perhaps Abula is jealous of Mirabel. Mirabel might be getting too much attention or praise lately, and Abula, feeling overshadowed, decides to get rid of her by kicking her out. In this scenario, Abula could be a rather petty character who can't handle someone else being in the spotlight.
Well, perhaps Harry Potter has grown frustrated with the Order's methods. Maybe he feels that they are too cautious or not taking enough risks in the fight against Voldemort. So, he decides to break away and form his own group with a more aggressive approach to dealing with the dark forces.
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.