There could be a secret society within the wizarding world that has been controlling things from the shadows for a long time. They don't want Harry to disrupt their hidden order and would do everything they can to stop him from becoming a master of politics. They might have their own dark magic and powerful allies to use against Harry.
The pure - blood supremacists could be the main antagonists. They might oppose Harry's ideas of a more inclusive and democratic political system as they believe in the superiority of pure - blood wizards.
It could be the Death Eaters who see Harry's new - found mastery of darkness as a threat to their own power and ideology. They might try to either recruit him or destroy him. Voldemort himself could be an antagonist, feeling that Harry is encroaching on his territory as the most powerful dark wizard.
Obviously, Voldemort could still be the main antagonist. He would be intrigued and threatened by Harry's new power, the lightsaber. Also, some Death Eaters might team up with dark wizards from other lands who are also after the power of the lightsaber.
The Ministry of Magic could also be antagonists. They are often very strict about magic use and might not approve of Harry's exploration of wandless magic. They could try to stop him, fearing that it would disrupt the established order of magic that is mostly wand - based. The Ministry officials might see Harry as a troublemaker and try to put him under surveillance or even arrest him if they think he's getting too powerful in wandless magic.
There could be a powerful dark entity that has been awakened by Harry Potter's newfound mastery of fire and darkness. This entity could be an ancient evil that was once sealed away and now wants to use Harry for its own dark purposes or simply destroy him as he is a potential obstacle.
Well, it could be about Harry using his skills and influence in the wizarding world to reform the political system. Maybe he starts by uniting different magical communities within Hogwarts, like the houses, and then expands it to the whole wizarding society.
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.
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.
Voldemort could still be the main antagonist. He might see Harry's connection to the monsters as a threat to his own power and try to either use it for himself or destroy Harry to prevent him from uniting the monsters against him.
Voldemort or his remaining Death Eaters could still be the main antagonists. They might try to take advantage of Harry's Two - Face situation. For example, they could try to manipulate his 'bad' side to turn against his friends and the wizarding world.
In this fanfiction, there might be a group of wizards who oppose the idea of mixing magic with the skills taught by Hephaestus. They could be the antagonists as they try to prevent Harry from using his new - found skills. They might believe that this new form of magic is dangerous or against the traditional ways of the wizarding world. They could form an organization to oppose Harry and try to disrupt his training and any plans he has to use his skills for good.