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.
Perhaps it could be a group of pure - blood supremacists who are against Snape's more inclusive ideas. They might see Snape's second chance as a threat to their traditional ways. They could try to undermine his efforts at Hogwarts, like spreading rumors about him or trying to disrupt his classes. They would be the main antagonists as they fight against the positive changes Snape is trying to make.
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.
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.
Well, perhaps in this fanfiction, Snape gets a second chance to change his relationship with Harry. Instead of being so cold and harsh, he could mentor Harry properly from the start. He might reveal his true intentions and past to Harry, and together they could fight against Voldemort in a more united front, with Snape using his knowledge and skills in a more positive way.
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 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.
The main antagonist could be Dumbledore. Since Harry has joined Voldemort, Dumbledore would see Harry as a threat to the good of the wizarding world. Dumbledore would try to bring Harry back to the right side, and they would end up in conflict.
Another option is a group of anti - magic fanatics. They view Percy's wings as an abomination because they are a result of magic. They hunt Percy down, believing that by getting rid of him and his wings, they are protecting the world from the 'evils' of magic.
It could still be Hawk Moth. Since Nathalie has been working with him, getting a Miraculous might make her a threat to his plans, so he could turn against her and become the main antagonist.
It could be a long - lost relative of an old enemy. For example, if there was a powerful dark wizard that Natsu defeated in the past, this relative might come seeking revenge and be powerful enough to kill Natsu.