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.
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.
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.
It could be some of the traditional antagonists from the original story, like the Volturi. They might see the baby as a threat to their power or the order they've established in the vampire world.
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.
There could be a dark spirit that haunts the woods. It's jealous of Bella's presence and tries to drive her out. Another antagonist could be a neighboring landowner who wants to expand their property into the woods where Bella lives.
The Volturi could be antagonists. Since they are the vampire authority, they might see the baby as a threat to the vampire world order. They could be worried about the baby's mixed heritage and its potential power, so they might try to eliminate the baby and even Bella and Klaus. Another possible antagonist could be some of Klaus's old enemies. They might see the baby as a way to get revenge on Klaus, either by kidnapping the baby or causing harm to it and its parents.
The main antagonists could be the traditional Naruto villains like Orochimaru or the Akatsuki members. They might target Naruto's sister as a way to get to Naruto, seeing her as a weakness or a way to manipulate him.
The Akatsuki could be the main antagonists. Since Naruto is an important target for them, and if Kurenai is now his sister, they might see her as an obstacle in their way. They could try to use their various powers and strategies to either capture Naruto or cause harm to both him and Kurenai.