Perhaps an escaped dark wizard from Azkaban. He might see Hermione as a threat in the future to his plans, so he turns her into a baby to stop her from growing up and becoming a powerful witch. He could be lurking in the shadows, trying to prevent her friends from reversing the spell.
It could be a jealous student who wants to undermine Hermione's intelligence. They might have cast the spell to turn her into a baby thinking it would be a great way to get her out of the way. This student could be someone who is always overshadowed by Hermione in class and exams.
It could be an old rival of the Malfoy family. Maybe a wizard whose family was wronged by the Malfoys in the past. This wizard has been biding his time, waiting for the perfect opportunity to strike. Turning Draco into a baby is his way of getting revenge on the family, making them look weak and powerless.
The Akatsuki might be the main antagonists. They might think that a baby Sasuke is easier to capture and they could use him for their own purposes, like extracting his Sharingan or something else related to his power. They would be a great threat as they are powerful and cunning.
Well, one possible plot could be that her friends have to take care of her as a baby. Harry and Ron might struggle to figure out how to keep her safe and fed. They could search for a way to reverse the spell that turned her into a baby.
It could be Team Rocket as always. Since Ash is now a Pikachu, they might see it as an easy target to capture. They could have new gadgets specifically designed to catch Pikachu - Ash, and constantly try to get their hands on him throughout the fanfiction.
Another possibility is an ancient evil that has been awakened. This entity might see Hermione as a threat because of her potential power. So it curses her to stop her from growing stronger. In this case, the antagonist would be very powerful and mysterious, and Hermione and her friends would have to do a lot of research and exploration to find a way to defeat it.
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.
It could be Orochimaru. He might see Naruto's new form as an interesting experiment or a threat to his plans. He could try to capture Naruto to study her newfound female - related powers or use her in some dark jutsu.
The antagonist could be an evil wizard. He might be jealous of the oc's natural magical abilities and decides to turn the oc into a kitten to weaken them. Another option is a vengeful fairy. Maybe the oc accidentally stepped on her flower garden, and in retaliation, she uses her magic to transform the oc into a kitten. Or it could be a power - hungry sorceress who wants to use the oc's new kitten form for some dark magic experiment.
The main antagonist could be the one who turned Percy into a monster in the first place. Maybe it's a vengeful god who feels Percy has wronged them in some way and decides to punish him by turning him into a fearsome beast.