A vengeful spirit could also be the main antagonist. This spirit could be related to someone Michael has killed in the past. It notices Michael's strange connection with Bella and decides to use her as bait to get revenge on Michael, putting Bella in great danger and forcing Michael to face his past actions.
Perhaps a powerful vampire clan. In the world of Bella, vampires are a big part of her universe. A vampire clan could see Bella's association with Michael as a threat to their plans or territory. They might try to separate them and even kill Michael, believing he is a mortal who doesn't belong in their supernatural world.
It could be another serial killer who is jealous of Michael's newfound 'interest' in Bella. This killer might see Bella as a prize and wants to take her away from Michael, leading to a battle between the two killers over her.
One idea could be that Michael Myers becomes strangely protective of Bella. Maybe he sees her as an innocent in a world full of chaos like his own, and he starts to lurk around her to keep other threats at bay. For example, when some local bullies start to harass Bella, Michael takes care of them in his own menacing way.
Perhaps a corrupt law enforcement officer. He doesn't want Jake to get too close to Michael Myers because he has his own secrets related to Michael's past killings. He tries to stop Jake from uncovering the truth and ends up being a major obstacle for both Jake and Michael.
It could be another powerful enchantress who is jealous of Bella's powers and wants to steal them for herself. She might constantly try to set traps for Bella and undermine her at every turn.
Maybe there's an evil sorcerer or wizard who sees Bella's weakness as an opportunity. He or she might want to use Bella's weakness to gain something, like access to a powerful artifact that only Bella can reach because of her limitations. The sorcerer would be constantly trying to manipulate her into doing his bidding.
A jealous student could be the main antagonist. This student might have a crush on Alice and see Bella as a threat to their relationship. They could start spreading rumors about Bella, trying to make her look bad in front of Alice and the other students.
The Volturi Queen herself could be the main antagonist. If she views Bella as a threat to her power or the Volturi's way of life, she would be a very powerful foe. She could use her influence over the Volturi guards and her own powers to try and stop Bella.
A rival alpha wolf could be the main antagonist. This rival might see Bella as a weakness in the alpha wolf's leadership or might want Bella for himself. He could constantly try to undermine the relationship between the alpha wolf and Bella and start battles with the alpha wolf's pack.
A powerful wizard who wants to harness Bella's destructive power for his own evil purposes could be the main antagonist.
It could be one of Kol's vampire siblings who is jealous of his relationship with Bella. Maybe they think Bella is a distraction for Kol and they want to break them up to keep Kol focused on family matters or their own power plays within the vampire world.
Another possible antagonist could be a new vampire character. Maybe a powerful and jealous vampire who also has an interest in Aro. This vampire could see Bella as an obstacle to their own desires and start causing trouble for her and Aro's relationship. They could spread rumors, try to frame Bella for crimes, or directly attack her to drive her away from Aro.