Who would be the most likely antagonist in Bella Swan and Chris Halliwell fanfiction?

One idea could be that Bella, with her experience in the supernatural world from the Twilight series, somehow crosses paths with Chris Halliwell from Charmed. They could team up to fight a new kind of evil that combines the dark magic from the Charmed universe and the vampire - werewolf related threats from Bella's world. For example, a coven of vampires who have learned some dark spells.

It could be a dark wizard. Dark wizards are often associated with using dark arrows as weapons in fanfiction. They might have a vendetta against Chris for some reason, like he unknowingly disrupted one of their dark magic experiments.

Alice Cullen can be a very popular character. Since she can see the future, in a 'reborn as bella swan' scenario, her interactions with Bella could be really different. Maybe she tries to guide Bella in a new way based on her visions. Also, Carlisle Cullen is popular. His father - figure role towards Bella might be explored further in these fanfictions.

Another possible antagonist could be the CCG in the 'Tokyo Ghoul' universe. If they were to somehow cross paths with the 'Death Note' world, they might see the power of the Death Note as a threat and try to take it. This would put them in direct conflict with both Kaneki and Light, as Light wouldn't want to lose his tool for 'justice' and Kaneki wouldn't want the CCG to gain more power to hunt ghouls. So, in this complex crossover situation, the CCG could end up being an antagonist.

It could be a jealous vampire who doesn't like Bella's new - found power. Maybe this vampire wants to be the most powerful being in the area and sees Bella as a threat.

A powerful werewolf could be the main antagonist. In the fanfiction world, werewolves and vampires often have a tense relationship. If Bella's sister does something that angers the werewolves, like stealing from their territory or harming one of them accidentally while doing her criminal deeds, the werewolf pack leader could become the main antagonist, vowing revenge on the sister and by extension, Bella and the Cullens.

Vlad Masters could be a likely antagonist. He might see Kim as a threat to his plans regarding Danny. Since he's always looking for ways to get Danny on his side or out of the way, he could try to use Kim as a pawn. Maybe he tries to manipulate Kim into thinking Danny is the bad guy, or he uses his ghost powers and wealth to create problems for both of them.

Perhaps an ancient evil entity that was once sealed away but is now breaking free. It wants to destroy all the good in both of their worlds, and Feyre and Aelin are the only ones who can stop it. The entity would be extremely powerful, with abilities that can counter both Feyre's and Aelin's powers to some extent.

Edward is still very popular. Since he and Bella are now both vampires, their relationship takes on new aspects that fans like to explore. Another popular character is Alice. Her ability to see the future could be really interesting in a story where Bella is a vampire. She could foresee events that involve Bella and help her avoid danger or make important decisions.

Another theme is self - discovery. The original characters usually go through a process of learning about their own powers and identities, and Chris might play a role in that. They could discover new magical abilities together and figure out how to use them.