Another possible antagonist could be a group of dark sorcerers. They might sense the unique power within Loki's baby daughter and want to either corrupt her and turn her to their side or use her power for their own evil purposes. They would send their minions to try and capture her and bring her to their lair.
In some fanfictions, a power - hungry Asgardian noble might be the antagonist. They could be jealous of Loki having a daughter with potential power and see her as a threat to their own position in Asgard. So they would plot against the baby daughter, spreading false rumors about her or trying to frame her for something she didn't do.
The Frost Giants could be a main antagonist. Since Loki has a history with them, they might see his baby daughter as a way to get revenge on Loki or gain some advantage in the power struggle with Asgard. They could try to kidnap her to use her as a bargaining chip.
Maybe some dark elves or other villains from the Marvel universe. They could see the baby as a valuable asset or a threat, and try to harm the baby or use it against Loki. Since Loki is now responsible for the baby, he has to fend off these threats to keep the baby safe. For instance, the dark elves might think the baby has some special power and they want to harness it for their own evil purposes.
One possible plotline could be the baby daughter inheriting Loki's mischievous nature. She might play little tricks on the Asgardians, like changing the color of Thor's hammer for a day. Another could be her growing up in a world that is sometimes hostile to her because of her father's reputation, and she has to prove herself to be different. And perhaps she discovers a unique power within her that no one in Asgard has ever seen before.
The main antagonists could be those who envy her wisdom. They might be jealous of the attention she gets or the power that her wisdom gives her. So, they try to undermine her at every turn, spreading false rumors about her or trying to make her look foolish in front of important people.
It could be the old enemies of Jack Sparrow. They might see his daughter as a way to get revenge on him. For example, those he has double - crossed in the past.
The descendants of the Death Eaters would make great antagonists. They might still hold a grudge against Sirius Black for his actions against Voldemort and his followers. They could target his daughter to seek revenge or to prevent her from uncovering any secrets that Sirius might have known about them.
The Penguin could be a main antagonist. He might see the daughter as a new threat to his criminal operations in Gotham. Since she has the combined skills of Batman and Catwoman, she could easily disrupt his illegal business, like his smuggling operations.
It could be a new group of rogue ninjas who oppose the ideals of the Hidden Leaf Village. They might target Kakashi's daughter because of her connection to Kakashi and Naruto, seeing her as a symbol of the village's new generation of strength.
The Foot Clan could be a main antagonist. They might see the baby sister as a way to get to the turtles, either by kidnapping her or using her to blackmail them. Another could be Shredder himself if he returns. He would surely not pass up the opportunity to use the baby sister against the turtles. And Baxter Stockman could also be an antagonist. He might be interested in using the baby sister for some of his mad - scientist experiments.
There could be a rogue Saiyan who has different ideals from Vegeta. This Saiyan might think that the baby should be raised in a more traditional, warrior - like way and try to take the baby from Bulma and Vegeta. Or, an evil scientist who wants to experiment on the baby's unique Saiyan - human DNA could be the main antagonist, creating all kinds of challenges for Vegeta and Bulma to protect their child.
It could be some rogue shinigamis who oppose the idea of rukia and ichigo having a child together. They might see the baby as a threat to the existing power structure in the Soul Society.