Maybe a group of pirates who have heard rumors of a special baby that is part of the royal family. They think they can ransom the baby or use it to gain some sort of power. They start to lurk around the shores and eventually make an attempt to kidnap the baby, which Ariel and Eric must prevent.
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.
Maybe the plot involves the baby getting kidnapped by an evil sea witch who wants to use the baby's unique heritage for her own dark purposes. Ariel and Eric then embark on a dangerous adventure to rescue their baby. Along the way, they meet various sea creatures and humans who either help or hinder them. In the end, they manage to save their baby through their combined strength and love.
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.
Often, in fanfiction, Ursula can still be a main antagonist. She might come back with a new plan to break up Ariel and Eric, maybe using a different form of magic or disguise this time.
One challenge could be the reactions of their families. Their families might have different expectations or concerns. Another could be the fear of the unknown, like potential problems during the pregnancy.
Their baby could look very much like a normal human baby at first glance, but on closer inspection, it has some mermaid - like qualities. For example, its ears might be slightly pointed like Ariel's, and when it's in the water, it seems to move with an unusual grace. It has a beautiful face with a combination of Ariel and Eric's best features, and its skin has a soft, almost iridescent sheen in certain lights.
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.
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 Orochimaru. He might see the baby versions of Team 7 as perfect subjects for his experiments. He could try to kidnap them to use their unique chakra or potential for his own dark purposes.
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.