A sly fox from an old - fashioned fox gang could be the main antagonist. This fox might see the baby fox as a threat to their way of life or might want to use the baby fox's innocence for their own devious plans.
There could be a jealous rival animal. For instance, a rabbit who is jealous of all the attention the baby fox is getting. This rabbit could then start spreading false rumors about the baby fox and try to turn other animals against it in the story.
A possible main antagonist could be a corrupted version of Bellwether. After her time in prison, she could find a way to tap into the power of darkness. She wants revenge on Zootopia and uses the Heartless to cause chaos. She would be a devious antagonist as she already knows the ins and outs of Zootopia.
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.
Another possibility is that there is a group of anti - jinchuriki people who see Naruto as a threat because of his connection to the giant fox. They might try to kill Naruto to get rid of the 'giant fox problem'. Their leader could be the main antagonist, someone who has a deep - seated hatred for anything related to the tailed beasts.
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.
It could be an old vampire enemy of Damon's. Maybe someone from his past who has a vendetta against him and sees the baby as a way to get revenge. This antagonist could try to kidnap the baby or cause trouble in their lives.
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.
It could be another dark mage who sees the potential in baby Natsu and wants to use him for evil purposes. This mage might view Zeref as an obstacle in getting to Natsu.
It could be a dark wizard who wants to use the baby for some evil purpose. Maybe the baby has a special power that the dark wizard desires to harness for his own gain. So he constantly tries to steal the baby from Lisanna and Bixlow.
Slade could also be a possible antagonist. He's always been interested in causing trouble for the Titans. The baby would be a new target for him. He might try to kidnap the baby to manipulate Raven and Beast Boy into doing his bidding. Or he could try to use the baby to gain access to some powerful artifacts or secrets that are related to the Titans.