Who would be the main antagonist in a Harry Potter and God of Thunder fanfiction?

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.

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.

It could be a new, dark demigod who has been corrupted by dark magic. This demigod might envy Percy's relationship with the gods and the God of Thunder in particular. He or she could be gathering an army of monsters and using dark powers to try and take over the world of the demigods and gods alike. They would be a formidable foe as they understand the ways of demigods but also have access to dark powers that are not easily countered.

It could be Orochimaru. He's always been after power and Naruto's new God of Thunder powers would be very appealing to him. He might try to steal Naruto's powers or find a way to control him.

It could be a power - hungry alien leader who wants to take over Hogwarts and use its magic for their own empire.

A dark wizard from the Harry Potter universe could be the main antagonist. Maybe a follower of Voldemort who escaped the final battle and found his way to Byakko's world. He sees the power of Byakko and wants to use it for his own evil purposes. He could try to enslave Byakko and use its power to take over both the wizarding world and Byakko's realm.

In a 'Harry Potter trained by Goku' fanfiction, the main antagonist could be an evil wizard who has allied himself with dark forces from other dimensions. This wizard could see Harry, with his new skills from Goku's training, as a major threat to his plans of world domination. He might have powers that are a mix of dark magic and some sort of energy manipulation similar to ki. Harry would then have to use his combined magic and Goku - trained skills to take on this powerful and devious enemy.

It could be Voldemort who somehow gets his hands on Oscorp technology and uses it to make his dark magic even more powerful. He would be a formidable foe for both Harry and Peter, as he has his own army of Death Eaters and now enhanced powers. Harry would use his magic and Peter his superpowers to fight against Voldemort's new - found strength.

Most likely, Medusa would be the main antagonist. Her ability to turn people to stone makes her a very dangerous foe for Harry Potter and the other characters in the wizarding world.