It could be a follower of the God of Death who misinterprets the God's will and tries to bring chaos to the wizarding world. They might see Harry as an obstacle to their plans and try to eliminate him.
Maybe there is a dark wizard from the past who was once in league with the God of Death and has been resurrected in the fanfiction. This wizard has his own agenda to rule the wizarding world using the power of the God of Death and will stop at nothing to achieve it, making him a major antagonist.
It could be a power - hungry Death Eater who didn't like how Voldemort was running things and decides to take over after his defeat. This character might be even more cruel and calculating than Voldemort, using dark magic in new and terrifying ways.
A group of rogue Death Eaters could be the main antagonists. They think that Voldemort has gone soft by teaming up with Harry. So they break away and form their own more extreme and violent group. They start attacking both the wizarding world and anyone associated with Harry and Voldemort's new alliance. Harry and Voldemort must then fight against their former allies who have become a greater threat than ever.
It could be a dark fairy who is jealous of the Fairy King's alliance with the wizarding world. This dark fairy wants to destroy the connection and take over both the fairy realm and the wizarding world.
It could be a rogue Death Eater who decides to carry on Voldemort's legacy in a more extreme way. This Death Eater might have some new and terrifying dark magic up their sleeve.
Perhaps an old follower of Voldemort who was in hiding. This person finds out about the Eyes of Death and believes that with their power, he can bring back Voldemort's legacy even stronger. He becomes the main force against Harry and his friends throughout the fanfiction.
It could be Voldemort again, but in a more powerful and death - centered form. Maybe he has found a way to fully embrace the power of death and become an even greater threat. Since Harry is the champion of death, it would be a battle of two opposing forces related to death.
It could be a rogue Death Eater who wants to take over Voldemort's position and sees Harry and Bellatrix's new - found alliance as a threat. For example, a Death Eater who was always jealous of Bellatrix's closeness to Voldemort.
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.