In some cases, a human antagonist could work. For example, a jealous classmate who is envious of Bella's sister's relationship with a popular boy. This could lead to a lot of petty but still impactful drama at school and in the community.
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.
It could be demons. Demons are often the main antagonists in the Supernatural universe, and they might target Bella because she is related to the Winchesters. They could try to use her as a bargaining chip or a way to get to the brothers.
Jacob could also make a great pairing. He has his own relationship with Bella, and his werewolf nature could create a lot of drama when interacting with the vampire sister. They might start off as enemies but gradually find common ground, like their shared love for Bella in a way, and then develop a deeper relationship.
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.
In this fanfiction, there might be some werewolves who view the baby as an abomination since it's part vampire. They could try to harm the baby, which would put Bella and Stefan in a difficult position to defend their child.
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.
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.
The Volturi could be antagonists. In the Twilight universe, they are the powerful vampire coven that enforces vampire laws. They might see the baby as a threat to their power or a violation of their rules. They could try to eliminate the baby or control Bella, Edward, and their family.
The Volturi could be antagonists. They are the powerful vampire coven that enforces vampire laws. Since Bella's transformation might not follow the normal protocol, they could see her as a threat to their order. They might come after her to either destroy her or bring her under their control.