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 a dark wizard who wants to control the dragons for his own evil purposes. He might see Harry as a threat because Harry is forming a bond with the dragons and teaching them to resist dark magic.
It could be a powerful wizard who sees Harry's transformation as an abomination and wants to destroy him. This wizard might believe that Nagas are dark creatures and that Harry, by becoming one, has crossed a line into the dark side of magic.
In some cases, it could be other dark wizards or witches who are envious of Harry's new power. They might want to steal his Obscurial power for themselves or use it to gain dominance in the dark wizarding underworld. These antagonists might be minor characters in the original series but could be developed more in the fanfiction to create new and interesting conflicts for Harry.
It could still be Voldemort. He might see Harry as an abomination as a cyborg, a perversion of the pure wizard blood. He would then be more determined than ever to destroy Harry, fearing that his new form gives him an edge in the fight against the dark arts.
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.
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.
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.
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.
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.
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.