Another possibility is that the Ministry of Magic turns against Harry. They might see his 'wasted' behavior as a threat to the stability of the wizarding world. They could start to spread false rumors about him and try to arrest him. In this case, the Ministry, with its strict rules and bureaucracy, becomes the main force that Harry has to fight against, just like how he had to fight Voldemort in the original series.
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 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.
Well, it could be about Harry getting really frustrated with all the pressure at Hogwarts and making some bad decisions that lead him down a dark path for a while. Maybe he starts skipping classes and getting into trouble with the professors more often.
The antagonist could be a jealous Slytherin student. They might see Harry's new snake as a threat to their own status in Slytherin house. So they try to steal the snake or harm it in some way.
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.
Voldemort could still be the main antagonist. Even with a new wand, Harry is still a threat to him, and Voldemort would be eager to get his hands on such a unique and powerful wand.
It could be Draco Malfoy. He might make fun of Harry for his new fitness obsession, thinking it's a silly Muggle - like pursuit. And this could lead to a series of confrontations between them.
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 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 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.