Another possible antagonist could be some dark wizards who are trying to use Harry's new identity for their own evil purposes. They might try to manipulate him or use his connection to the Blacks to gain access to powerful magic or influence within the wizarding world.
There could also be a trainer who is jealous of Harry's unique abilities, being reborn from the Harry Potter world. This trainer might try to undermine Harry at every turn, stealing his Pokemon or sabotaging his battles, much like Draco Malfoy was often in conflict with Harry in the original Harry Potter series.
Voldemort could still be a main antagonist. His dark magic and thirst for power would put him at odds with Harry, especially if he sees Harry's connection to Black Widow as a threat. Another could be some of the more traditionalist wizards who don't trust Harry because of his strange heritage.
The Ministry of Magic could be the antagonists. Since Harry is in a gang, they might see him as a threat to the order of the wizarding world. They could send Aurors after him and his gang mates, trying to arrest them and break up the gang.
An ancient and malevolent spirit that was once trapped in the black puddle but has now been freed could be a major antagonist. This spirit wants to take over the queen's body and use her connection to the puddle to spread chaos across all the realms. It constantly whispers dark thoughts into the queen's mind, trying to make her lose control and become a puppet for its own evil plans.
It could be the Saiyans. In the Dragon Ball universe where Frieza exists, the Saiyans are often his enemies. Even if Harry is reborn as Frieza, the Saiyans' natural strength and warrior spirit would pose a great threat to him. They might not understand his new - found combination of magic and Frieza's powers and see him as an even greater danger.
The Thalmor could be great antagonists. Their strict control over magic and their anti - human beliefs could put them at odds with Harry. They might see his different form of magic as a threat to their power in Skyrim.
Some of the more traditional pure - blood families in the wizarding world could be antagonists. They might not approve of Harry, a half - blood, being involved with Pansy, and they could try to stop the pregnancy or break up the couple for the sake of 'pure - blood' ideals.
Voldemort or his followers could still be antagonists. They might not like the idea of Harry being under Euryale's protection as it could make him more powerful. Another possibility could be some creatures from Euryale's world who are against her taking in an outsider like Harry.
Perhaps the very magic that made Harry a god has a sort of negative side - effect. This could manifest as a shadowy entity within Harry that tries to take over his mind and use his powers for evil. So, in a way, this internal struggle could be the main antagonist in the story.
It could be other members of the royal family who see Harry as a threat to their power or position. They might try to undermine him and prevent him from claiming his rightful place as king.