It's also possible that some of the werewolves who are not as accepting of change could view Bella's new mermaid form as something abnormal and a potential danger to their pack. So they could act as antagonists in the story, causing trouble for Bella as she tries to adjust to her new life.
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.
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.
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.
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.
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.
Another possibility is that there is a mysterious figure who has some sort of vendetta against the family. This figure could be causing strange and unexplained events to occur around the baby, like making the baby sick in a way that doctors can't figure out at first. This would then be a source of conflict as Bella and Sam try to protect their baby from this unknown threat.
In some cases, it could be other time - traveling vampires or supernatural beings who have their own agendas. They might not want Bella to interfere with the past as it could disrupt their own plans or power bases. These antagonists could be more shadowy figures, working behind the scenes to thwart Bella's efforts.
It could be their families if they are against the idea of Bella and Embry having a baby. For example, if Bella's family has high expectations for her and doesn't think Embry is a suitable partner, they could try to interfere and cause problems.
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.
There could be a friend of Bella's, like a human friend who starts to suspect that something is different about Bella. This character could add an interesting dynamic as they try to figure out Bella's secret and support her at the same time.
Maybe there are some traditionalists in their world who don't approve of their relationship or the pregnancy. These people could be the antagonists, constantly trying to break them apart or cause problems for them.