It could be Voldemort still. Even with Harry having the power of a dragon god, Voldemort would be a major threat. He might be envious of Harry's new power and try to steal it for himself.
Perhaps there could be an ancient dragon hunter or a group of them who see Harry as an abomination because of his connection to the dragon god. They might want to kill him to prevent the dragon god's power from spreading in the wizarding world.
It could be Voldemort and Loki. Voldemort, being the main villain in the Harry Potter series, is power - hungry and wants to rule the wizarding world. Loki, from the God of Thunder universe, is known for his trickery and his own desires for power. They could team up, thinking that they can use each other to achieve their goals. Voldemort could be interested in Loki's ability to deceive and Loki could see the potential in Voldemort's dark magic. They would be a formidable pair against Harry Potter and Thor.
Another possibility is a dark wizard who has been secretly studying the power of the God of Lightning for years. This wizard wants the power all for himself and sees Harry as an obstacle. He could be a former Death Eater who has broken away from Voldemort's group to pursue this power on his own. He would use his knowledge of dark magic and his own minions to try and take the power from Harry.
It could be a dark wizard who also has some form of speed - related power but uses it for evil. Maybe he's jealous of Harry's newfound speed abilities and wants to steal them for himself. He would constantly be chasing Harry and trying to disrupt his life at Hogwarts.
Maybe a group of dark wizards who are jealous of the power of the dragon knights. They form an alliance to destroy the dragon knights and anyone associated with them, making Harry and his newfound friends their main targets.
Perhaps a group of anti - dragon fanatics within the wizarding world could be the main antagonists. They see the dragon as a dangerous and uncontrollable creature and will do anything to get rid of it, even if it means going against Harry.
Maybe it's an organization that believes the dragon's blessing is an abomination to the wizarding world's natural order. They see Harry as a threat and will stop at nothing to eliminate him and the power he has received. They could be a secret group within the Ministry of Magic that has its own radical ideas about magic and purity.
There could be a group of pure - blood wizards who believe that dragons are beneath them and that Harry is tainting the wizarding world by associating with them. They would try to stop Harry from his dragon - related activities, and they might use underhanded tactics like casting spells on him or trying to get him expelled from Hogwarts.
It could be another wizard who wants to use the dragon for evil purposes. This wizard might try to stop Harry from taming the dragon so that he can have it for himself.
In some cases, it could be a jealous Hogwarts student. This student might be envious of Harry's special bond with the dragon. They could try to undermine Harry and the dragon's relationship, maybe by spreading false rumors or trying to get the dragon in trouble with the school authorities. This student could be working with some dark forces indirectly, as their envy makes them vulnerable to being influenced by the wrong side.
It could be a dark wizard who wants to control the holy dragon for his own evil purposes. He might see the dragon as a weapon to take over the wizarding world.