It could be some jealous Fae who don't like Bo having a half - human baby. They might see it as a threat to the Fae purity or something.
Maybe a group of scientists who had been secretly studying Lauren and want to get their hands on the baby to study its genetics. Since Lauren has been involved in some unique medical and scientific things, they think the baby could hold the key to some great discovery.
There could be a dark Fae entity that feeds on the innocence of new - borns. It senses the power within Bo and Lauren's baby and tries to take it away. This would create a great conflict for Bo and Lauren as they fight to protect their child from this menacing force.
One possible plot could be that Bo and Lauren face challenges in raising the baby while also dealing with the supernatural elements in their world. Maybe the baby has some unique powers that attract unwanted attention from evil forces.
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.
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.
One possible antagonist could be an evil scientist who wants to study the baby's unique genetic makeup. Since Shadow and Blaze are both special characters, their baby would likely have an interesting genetic combination that the scientist could use for his own malicious purposes.
Maybe an old acquaintance from their past who has a grudge against them. This person could spread rumors about them being unfit parents just because they are in a same - sex relationship. And that would create a lot of problems for Emily and Alison as they prepare for the baby.
It could be Bowser. He might see the new baby as a way to get to Daisy and Luigi. He could try to kidnap the baby to use as leverage against them.
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 some secret organizations that want to study the baby because it's a hybrid of Ben and Gwen's powers. These organizations might not have evil intentions at first, but their methods of trying to get the baby could make them antagonists as they go against Ben and Gwen's wishes to protect their child.
The other gods who oppose the union of a mortal (or demigod in Percy's case) and a goddess could be antagonists. They might see the baby as an abomination or a threat to the order of the divine world. For example, some of the more traditional and strict gods could try to harm the baby or separate Percy and Hestia.
The antagonists could be forces of dark magic. They might sense the power within the baby and want to either corrupt it or use it for their own evil purposes. For example, there could be a sorcerer who wants to steal the baby and raise it to be his minion. Also, some neighboring kingdoms that see Camelot's new heir as a future threat might send spies or assassins to target the baby.