It could be a criminal mastermind who has a vendetta against Beckett or someone close to her. This antagonist might see the baby as a way to get back at Beckett, either by using the baby as bait or harming it to cause her pain.
Perhaps it's a group with a dark agenda. They could be involved in illegal activities like human trafficking or some sort of black - market dealing, and the baby is just a part of their latest scheme. They might be well - organized and have resources that make it difficult for Beckett to track them down.
It could be a criminal with a vendetta against Beckett. They might abduct the child to get back at her for putting their associates in jail in the past. This antagonist would be calculating and always one step ahead, making it a real challenge for Beckett to rescue the child.
One possible plot could be that Beckett embarks on a dangerous journey to find the abducted baby. Along the way, she uncovers a complex web of secrets and has to outwit the kidnappers. Maybe she teams up with an unexpected ally who has some inside knowledge about the abduction.
It could be a corrupted Ministry of Magic official. This official wants to use the Muggle world for his own gain, and he sees Kate Beckett as a threat. He teams up with some dark wizards and tries to stop Harry and Kate from uncovering his plans. This would create a complex web of intrigue as Harry and Kate have to fight against not only dark magic but also the corruption within the wizarding world.
Another vampire family that is jealous of the Mikaelson's power and influence. They see the baby as a threat to their future plans. They might try to kidnap the baby and use it as a bargaining chip to gain more power in the vampire world. They would be constantly plotting against the Mikaelsons to get their hands on the baby.
Danzo could be a great antagonist even when Naruto is a baby. He might see Naruto as a threat to his plans for Konoha's future and try to get rid of him secretly while making it look like an accident.
A powerful dark wizard could be a great antagonist. He might want to use the Scarlet Baby's unique energy for his dark magic. He could send his minions, like flying monkeys or shadow serpents, to capture the baby.
A human noble who doesn't approve of Ariel and Eric's union and especially doesn't like the idea of a half - human, half - mermaid baby. This noble could try to create problems for the family, like spreading false rumors or trying to get the baby separated from its parents. Another possibility is a sea monster that has been awakened by the birth of the baby, seeing it as an intrusion in its territory and trying to harm the baby and its parents.
An overzealous social worker who believes that Rizzoli and Isles are not fit to take care of the baby due to their dangerous jobs. This social worker could constantly be trying to take the baby away from them, creating a lot of conflict and drama in the fanfiction. Rizzoli and Isles would then have to prove that they can be great parents despite their demanding careers.
It could be an old enemy from their past, like a vengeful monster. Maybe a cyclops that they once defeated and now it wants to take revenge on them through their baby.
A group of rebels who have different ideas about how to handle the situation of the baby could also be the antagonists. They might think that Effie and Haymitch are too soft or too Capitol - influenced in their parenting of the baby. So they try to take the baby from them to raise it in their own way, believing it will be better for the future of the rebellion and the new society they are trying to build.