It could also be that some within their own community are against Fox and Krystal having a baby. Maybe there are some old - fashioned or prejudiced individuals who think their union is wrong and try to cause trouble for the new family.
Another possibility could be a new, mysterious race that is jealous of the baby's potential powers. They could try to kidnap the baby to study it or use its powers for their own evil purposes.
The old enemies of Fox, such as Andross' remaining followers could be antagonists. They might see the baby as a new target to get back at Fox for all the defeats they suffered in the past.
One possible plot could be that Fox and Krystal face challenges in raising their baby in a world full of danger. Maybe they have to protect their child from an old enemy who resurfaces, seeking revenge on Fox by targeting the new - born. They would then embark on a journey to keep their family safe while also teaching their baby the ways of survival.
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.