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.
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.
Well, it could be about Harry getting a chance to redo a crucial event in the wizarding world. Maybe he has the opportunity to save someone he previously lost, like Sirius Black. He could use his knowledge from the first time around to outwit Voldemort in a different way, leading to a more peaceful resolution in the wizarding world.
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.
In this fanfiction, it might be that the main antagonist is actually a part of Harry himself. His inner demons, so to speak. The 'wasted' state could bring out his insecurities and negative traits, and these could manifest as an internal struggle. So, in a way, Harry is his own antagonist as he has to overcome this self - destructive side of him.
It could be a new dark wizard who is jealous of Harry's new mastery. This wizard might try to steal Harry's knowledge or use it for his own evil purposes. He could be a former Death Eater who was thought to be dead but has now returned with a vengeance.
It could be a rogue wizard who is jealous of Harry's new cybernetic abilities. This wizard might think that Harry is polluting the purity of magic with technology and decides to stop him at all costs.
It could be Voldemort. He might see Harry's new snake form as a way to either use him for his own evil purposes or as a threat to his power. Since snakes are often associated with Slytherin and dark magic, Voldemort might try to control Harry.
It could be Voldemort. He might see turning Harry into an elf as a way to weaken him. Since elves are generally seen as lower in the wizarding hierarchy, Harry in this form would be less of a threat to Voldemort in the eyes of many wizards.
The main antagonist could be the Ministry of Magic if they view the succubus as a dangerous and uncontrollable creature. They might try to separate Harry from his familiar, and in the process, become the main force that Harry has to oppose.