There could be a new type of hybrid creature that is a result of the combination of the zombie virus and some supernatural element from Bella's world. This hybrid could be extremely powerful and unpredictable, and it sees both humans and zombies as a source of food or something to dominate, making it a very dangerous antagonist in the fanfiction.
The Volturi could be great antagonists. They might see the zombie outbreak as a threat to their vampire world order. They could try to wipe out all the zombies and anyone associated with them, including Bella if she has any connection to the zombies.
Obviously Naruto could be one of the main characters. He would bring his unique abilities and his never - give - up attitude. Another could be Rick from The Walking Dead. His leadership skills combined with Naruto's power would make for an interesting dynamic. They could learn from each other and lead the survivors in different ways.
She could interact with Rick. Rick is the leader of the group in The Walking Dead, and he would be very interested in Zoey if she has any skills or knowledge that could help the group survive. Maybe she has some medical knowledge, and Rick would want her to take care of the injured in the group.
Jacob and his werewolf pack could be antagonists. Since Bella has a history with Jacob and the werewolves don't really trust the vampires, they might oppose the wedding. They could try to disrupt the wedding to keep Bella from marrying Demetri, believing it's not in her best interest.
Perhaps the Volturi could still be the main antagonists. Even though Bella is now a mermaid, the Volturi's strict laws about the supernatural world might still put her in danger. They could see her mermaid nature as a threat to their control.
The other gods who don't trust Hades might be antagonists. They could see Bella as a threat because of her relation to Hades. For example, Zeus might be worried that Bella could one day try to overthrow him or cause trouble in the order of the gods.
Maybe there are hunters in the fanfiction. Hunters who are dedicated to eradicating vampires and see Bella, as someone being raised by Dracula, as a prime target. They would pose a constant danger to her and Dracula.
Bella could develop morally. Maybe at first, she is hesitant to do what it takes to survive, like stealing food from other groups. But as she faces more difficult situations, she realizes that sometimes tough choices are necessary for survival.
The Akatsuki could also be the main antagonists. They might see the power of raising the dead as a way to achieve their goals. For example, Pain could use the raised dead to spread fear and chaos, or to try and find a way to resurrect Nagato. And they would definitely be a formidable force against Naruto if they were after this power.
One idea could be that Bella, with her unique ability to attract vampires in her original story, has an unexpected reaction to the zombies in The Walking Dead universe. Maybe her scent or some hidden power she has confuses the zombies, leading to some comical or strategic situations for her group.