One possible antagonist could be Sabretooth. He might see Wolverine's newfound fatherly relationship with Peter as a weakness and try to use Peter to get to Wolverine. Sabretooth could be jealous of the bond Wolverine is forming and want to destroy it.
One possibility is that Wolverine discovers Peter in a difficult situation, like being an orphan after some tragic event in the X - Men universe. Wolverine, with his tough - exterior but kind heart, decides to take Peter under his wing. He could teach Peter how to fight and control any potential powers Peter might have, all while forming a father - son bond.
Maybe some new villains from the multiverse. As Loki and Peter start exploring different realms together, they could encounter beings who want to use Peter's powers for their own evil plans. These villains might be powerful magic - users or creatures that are not from Earth and have different motives and abilities.
The Ministry of Magic itself could be an antagonist. If they don't approve of Hermione's adoption due to her age or some bureaucratic reasons, they could try to take the baby away. They might think she's not fit to raise a child, which would create a lot of conflict in the story.
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.
In the Naruto world, it could be Orochimaru. He might be interested in Naruto's newfound Pokémon - related powers and try to capture him to experiment on him. Orochimaru is always after power, and Naruto's unique abilities could be very appealing to him.
Another possibility is that some rogue ninja who have a vendetta against Hanzo might target Naruto as a way to get back at Hanzo. They could see Naruto as Hanzo's weakness and try to harm or kidnap him, which would then lead to a series of battles and confrontations in the fanfiction.
The main antagonists could be the traditional enemies of Camelot like Morgana. She might see Merlin's act of adopting Mordred as a threat to her plans. Also, some of the knights or nobles in Camelot who are superstitious and believe Mordred is a danger just because of the prophecies could be antagonists. They might try to separate Merlin and Mordred or even harm Mordred.
Perhaps some powerful organizations or institutions. If Peter is trying to disrupt the status quo, these entities that benefit from the current system would be his antagonists. They might see him as a troublemaker and try to stop him. They could use their resources and influence to try and take Peter down, creating a lot of conflict in the fanfiction.
Perhaps there are some anti - Uchiha elements within the village that are not happy with Naruto adopting Sarada. They could try to undermine her training or create problems for Naruto and Sarada. They might spread false rumors or try to isolate Sarada from the rest of the village.
The Death Eaters could still be the main antagonists. They might see Fenrir's new alliance with Harry as a threat. Since they once used Fenrir for their dark deeds, they don't want him to turn good and expose their secrets.