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.
Well, one key plot development could be that Paul's phasing next to Bella reveals some hidden powers within her. Maybe it's a power that she didn't know she had and it becomes crucial in fighting off some common enemy in the fanfiction world, like a group of rogue vampires. Another aspect could be that Paul's action creates a bond between them that goes beyond the normal werewolf - human relationship in the Twilight - inspired fanfic universe.
A common theme is the struggle for acceptance. Bella often has to accept the new situation of Paul's imprint on her. Another theme is the change in relationships. Her relationships with other characters, like the Cullens or her human friends, may shift due to this new bond.
One popular 'bella phases and imprints on paul fanfiction' is 'Bella's New Path'. It explores Bella's different emotional phases and how they impact her relationship with Paul. Another good one could be 'The Imprint's Influence' which delves deep into how Paul's imprint on Bella changes their lives and the pack dynamics.
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.
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 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.
One common theme is self - discovery. As Bella phases into a wolf, she has to learn about her new self, her powers, and her place in the supernatural world.