In some cases, there could be a new vampire or a shape - shifter who enters the prison and has a vendetta against Edward or Bella. This character would then be a major antagonist, constantly trying to harm them and prevent them from achieving their goals of getting out of prison or finding justice.
Maybe there are some powerful and corrupt humans in the gang wars. They are not afraid of vampires and want to take over the territory that Edward's family has been protecting. These humans could be the main antagonists as they constantly create trouble for Bella and Edward.
Often, other vampire clans that oppose Bella's transformation can be the main antagonists. They might see her as a threat to the vampire world order. For example, a traditionalist clan that believes humans should not be turned so easily could try to harm Bella or drive her away from Edward.
In some fanfictions, other vampires outside the Volturi who oppose Bella and Edward joining the Volturi could be antagonists. They might fear that Bella and Edward's new alliance will give the Volturi too much power. These vampires could then plot to stop Bella and Edward from fully integrating into the Volturi.
It could be that some of the humans who have discovered the existence of vampires and the new baby become antagonists. They might form a group to hunt down the Cullens and the baby, fearing that the baby could be the start of a new vampire - human hybrid race that could overpower humans.
In some Bella and Jasper military fanfictions, the main antagonists could be rival military groups. These groups might have different ideologies or goals and be in direct competition or conflict with Bella and Jasper's unit. For example, they could be fighting over territory or resources. Another type of antagonist could be a power - hungry military leader within their own ranks who is willing to sacrifice the well - being of the soldiers for personal gain.
It could be other werewolves who oppose the union of a human and a werewolf. They might see the pregnancy as a threat to the purity of the werewolf bloodline or something against their traditional beliefs.
Often, the Volturi can be antagonists. They might not approve of Jasper and Bella's relationship for some reason, like they think it goes against vampire laws or traditions. Another possible antagonist could be a rogue vampire who has a vendetta against the Cullens and sees Jasper and Bella's relationship as a way to get to them.
In some cases, a powerful and rogue vampire who has his own agenda could be the main antagonist. This vampire might be after Bella for some special ability she has or he could have a vendetta against Rosalie. He would constantly be causing trouble for them, trying to disrupt their lives and relationships within the Cullen family.
It could be that an old enemy of Stefan's family decides to target Bella as a way to get back at Stefan. This enemy might be another vampire clan or a supernatural being with a long - standing grudge. They would do everything in their power to disrupt the relationship between Bella and Stefan, causing all sorts of problems and conflicts for the two of them in the fanfiction.
There could be a dark magic user. This person might be jealous of Bella's relationship with Paul or wants to use her for some dark magic ritual. So they put Bella in danger, and Paul phases to protect her. The dark magic user would then be the main antagonist in this fanfiction.