Perhaps it's a corrupted Hogwarts professor. In this fanfiction, a teacher could have been influenced by the dark side and decides to oppose Harry's new path. They could be using their position at Hogwarts to spread dark magic teachings or to manipulate students against Harry. They would be a more insidious antagonist as they are operating from within the very institution that is supposed to be a safe haven for Harry.
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.
It could be a new dark wizard or witch who rises to power. Maybe someone who was once in Voldemort's shadow but now sees an opportunity with Voldemort gone (in this new fanfiction scenario where Snape has a second chance). This character could oppose Snape's attempts to reform the wizarding world and bring about a new era of peace and magic.
It could still be Voldemort, but in a different form. Maybe he has a new power or a different plan this time. Since it's a new chance, he might have found a way to come back even stronger and more devious than before.
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.
It could still be Voldemort. Even though Harry has a second chance, Voldemort is still a major threat in the wizarding world. His presence and his Death Eaters would continue to be the main source of danger, and Harry would need to use his second chance wisely to defeat him once and for all.
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.
Ron could also take on the role of the main character. He has his own brand of courage and loyalty. He could be the one who, in a moment of desperation, unlocks the power that is Voldemort's bane. For example, he might use his family's old magic artifacts in a new and unexpected way.
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.
It could be Harry Potter himself. Given all the hardships he faced in the original series, a second chance would open up a whole new world of possibilities for him. He could use this chance to prevent some of the tragedies that occurred, like the deaths of his friends or mentors.