In this fanfiction, the main antagonist could be the Volturi system itself. The strict laws and hierarchy of the Volturi might be what Bella and Alice are up against. Even if they have good intentions, the Volturi's way of doing things could be the obstacle they need to overcome. For example, the Volturi might not trust Bella and Alice and try to imprison them or use them for their own purposes.
Perhaps there is a rogue vampire within the Volturi's ranks who doesn't want Bella and Alice there and tries to undermine their purpose. This vampire could be jealous of their relationship or afraid that they will expose some of the Volturi's secrets.
Well, it could be that Bella and Alice are on a mission to learn some secrets from the Volturi. They might be disguised or using their unique abilities to get close to the Volturi leaders. It could also involve some tense moments of almost being discovered and having to use their wits to escape tricky situations.
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.
The Cullens could potentially be the main antagonists. Since Bella is secretly a Volturi fan, the Cullens' views and actions that oppose the Volturi might be seen as obstacles in Bella's new - found admiration. For example, if the Cullens try to keep Bella away from Volturi - related things, they would be acting against her interests.
One possible motive is to rescue someone. Maybe a friend or a family member of theirs has been captured by the Volturi. Another could be to gain knowledge. The Volturi are very old and have a lot of secrets about vampire existence, so they might go there to learn more.
It could be a new vampire who has a vendetta against Bella for some reason. Maybe this vampire wants to take over the territory or is jealous of Bella's relationship with the Cullens.
In some fanfictions, they might face a series of challenges. For example, they could be wrongly accused of breaking vampire laws. The Volturi might be very strict and unforgiving in the fanfiction world. Bella and Alice would have to use their wits and powers, if any, to defend themselves. Maybe they would try to negotiate with the Volturi leaders like Aro, Caius and Marcus.
There could be other vampires that Bella meets during her time as a vampire before the Volturi. For example, some lone vampires who are also trying to stay out of Volturi's radar. They could become her allies or foes depending on their motives.
In the Twilight universe where this fanfiction is based, the Volturi kings are Aro, Caius, and Marcus. They are the leaders of the Volturi, an ancient and powerful coven of vampires.
There could also be other vampires or humans who play important roles. For instance, some fanfictions might introduce new vampires who are either allies or enemies of the Volturi and have a connection to Bella's situation. And there might be human friends of Bella who get involved in the story, like Jacob or her school friends, although their roles might be secondary compared to Bella and the Volturi.
Obviously, Bella and the members of the Volturi are the main characters. Bella, as we know from the original story, is a human who has a special connection to the vampire world. And the Volturi members, like Aro, Caius, and Marcus, play important roles as they are the powerful vampire rulers.