A group of dark magic - using reindeer could be the main antagonists. Maybe they were banished long ago for misusing their magic, and now they've returned. They target Santa Baby because they know she's important to Santa and the whole Christmas operation. They try to steal her magic to make themselves powerful enough to take over the North Pole.
An ancient snow - spirit turned evil could be the antagonist. This spirit was once a protector of the North Pole but was corrupted by a dark force. It sees Santa Baby as a threat to its new - found evil plans. It tries to stop her and the Guardians from saving Christmas by creating blizzards and other dangerous weather conditions all over the world.
One possible unique plot could be that Santa Baby gets lost in a time - warp and ends up in a different era. The Guardians have to team up to find her and bring her back. Along the way, they discover new magical elements and face off against an ancient evil that was awakened by Santa Baby's time - travel mishap.
It could be a powerful alien warlord who wants to use the dying reader's unique energy for his own evil purposes. Since the reader is in a vulnerable state, this antagonist sees an opportunity to gain power.
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.
Pitch Black would be a likely main antagonist. He could manipulate Jack Frost into turning dark, as he has a history of preying on the insecurities of the Guardians.
A powerful dark wizard could be the main antagonist. He might be jealous of the dragon rider's powers and the bond with the dragon, and wants to break that bond and steal the rider's powers for himself.
It could be a former ally of the Pokemon Master who has turned evil. Maybe they felt overshadowed by the Pokemon Master and now seek revenge. For example, they might have found a dark artifact that corrupts them and gives them the power to control other trainers' Pokemon.
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.