It could be the pure - blood supremacists. They might see the relationship between Harry, a well - known wizard, and the Veela Queen as an abomination because they believe in the purity of wizard blood and would not accept a union with a non - wizard being like the Veela.
There could also be some jealous Veela or wizards. Maybe there are other Veela who were interested in the Queen and feel threatened by Harry. Or wizards who were envious of Harry's relationship with such a powerful and alluring being. These characters could create all sorts of problems and obstacles for the couple.
The Death Eaters, if the story is set during the time of Voldemort. They would oppose anything that doesn't fit into their narrow view of the wizarding world order. A relationship between Harry and the Veela Queen would be seen as a threat to their idea of a pure - blood dominated society, so they would try to stop it.
Some jealous students at Hogwarts could also act as antagonists. They might be envious of Harry's newfound allure and use underhanded tactics to try and make his life difficult.
It could be Umbridge. If she discovers Harry's transformation into a veela, she might see it as something unnatural and try to use it against him. She could try to expel him from Hogwarts or make his life even more miserable than she already did in the original story.
Some dark magic users could be the antagonists. They might want to capture the kitten for some dark magic ritual or use the dark veela's power for their own evil purposes. They would constantly be plotting ways to get to the veela and the kitten.
Well, one interesting plot could be that the Veela Queen uses her charm to initially draw Harry in, but as they get to know each other, he discovers she has a kind heart beneath her alluring exterior. They might go on adventures together in the wizarding world, facing challenges like dark wizards who oppose their union.
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.
One possible antagonist could be a corrupt wizard in a high - ranking position in the Ministry of Magic's financial department. This wizard might be trying to cover up his misdeeds and would see Harry as a threat to his scheme.
Another possibility is that it's a character related to the Veela community. A Veela elder or a young Veela who is chosen for this special training. This character would bring the unique Veela culture and magic into the story, and their interactions with the wizarding world outside of the Veela community would be an interesting aspect of the fanfiction.
Scarecrow could also be a main antagonist. His fear - inducing toxins could be a real problem for Harry. Harry would have to use his magic to counter the effects of the fear toxin and also figure out a way to stop Scarecrow from spreading it all over Gotham.
It could be the other Titans who don't want a half - wizard, half - Titan like Harry to exist. They might see him as a threat to their pure Titan bloodlines or their power structure.
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.