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.
In some versions of this fanfiction, there could be a new vampire who has turned rogue and is leading one of the gangs. This vampire has a vendetta against Edward for some past reason and is using the gang wars to get revenge. So, this rogue vampire could be one of the main antagonists.
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.
It could be the Volturi. Since the baby is a vampire - human hybrid, the Volturi, who are very strict about vampire laws, might see the baby as a violation of their rules and try to eliminate it.
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.
One possible plotline could be Bella and Edward getting caught in the middle of a gang war while trying to protect their family and friends. Maybe they have to use their unique vampire and human skills to outwit the gangs.
The Volturi could be antagonists. In the Twilight universe, they are the powerful vampire coven that enforces vampire laws. They might see the baby as a threat to their power or a violation of their rules. They could try to eliminate the baby or control Bella, Edward, and their family.
The Ministry of Magic could be the antagonists. Since Harry is in a gang, they might see him as a threat to the order of the wizarding world. They could send Aurors after him and his gang mates, trying to arrest them and break up the gang.
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.