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.
Maybe there are some powerful and corrupt humans in the gang wars. They are not afraid of vampires and want to take over the territory that Edward's family has been protecting. These humans could be the main antagonists as they constantly create trouble for Bella and Edward.
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.
It could be a new dark wizard who wants to use Harry's daughter to get revenge on Harry. This wizard might be jealous of Harry's fame and power and think that by harming his daughter, he can destroy Harry's happiness.
One potential main antagonist could be a powerful pirate captain who sees Harry as a threat to his control over the seas. Maybe this captain has some dark magic powers as well.
There could be a new group of dark wizards or witches who are aware of Harry's status as a male carrier. They might see him as a threat to their plans for domination in the wizarding world and thus try to stop him or take whatever he is carrying for themselves. This new group could have different motives and powers compared to Voldemort's Death Eaters.
The Joker could be a main antagonist. He would be fascinated by Harry's magic and would try to use it for his own evil purposes or just to cause chaos. He might try to drive a wedge between Harry and Batman by using mind games.