There could be a jealous student. This student might be envious of Harry getting all the attention with his snake - related adventures. So, they would act as an antagonist by trying to undermine Harry's efforts in the snake fanfiction world. They could spread false rumors about snakes being dangerous or Harry's intentions being dark, causing trouble for Harry among his peers and teachers.
The Ministry of Magic could be the main antagonist. They might view Harry's fascination with Dementors as a sign of him turning to the dark side. So they try to stop him from his research and exploration of Dementors, fearing he'll use their powers for evil. After all, Dementors are associated with dark magic in the eyes of the Ministry.
There could also be new antagonists. For example, some of the more traditional wizarding families who are against the idea of a female Harry. They might see her as a threat to the old - fashioned ways of the wizarding world and try to undermine her.
Draco Malfoy could be a main antagonist. Since Harry becoming a prefect is a sign of prestige and power, Malfoy, who is always jealous of Harry, would try to undermine him. He might spread false rumors about Harry misusing his prefect powers or try to get him in trouble with the professors.
Maybe Voldemort or his followers could still be antagonists. Even though Harry is now a vampire, Voldemort might want to use his new state for his own evil purposes or simply view Harry as a nuisance that must be destroyed. Since Harry has changed, he might have new weaknesses that Voldemort could exploit.
Maybe a dark wizard who wants to capture the phoenix - Harry for his own evil purposes. This wizard could be after the unique powers that a human - turned - phoenix like Harry would possess. He might want to use Harry's phoenix form to gain immortality or to spread dark magic across the wizarding world.
It could be Voldemort still. Even if Harry becomes associated with death, Voldemort is the ultimate symbol of death and destruction in the wizarding world. Their battle could take on a new dimension as Harry now has a different relationship with death.
It could be a dark wizard who also has the ability to communicate with snakes and wants to use this power for evil. He might be jealous of Harry's connection with the snake and try to disrupt it or steal the snake for his own dark purposes.
The main antagonist could be a Slytherin student who has a vendetta against Harry. When they see Harry with the snake, which they believe should belong to a Slytherin, they start causing trouble for Harry. They might try to frame Harry for using dark magic with the snake or try to steal the snake to prove their superiority over Harry.
It could be a dark wizard who also has an interest in the snake. Maybe he wants to use the snake's powers for evil purposes, like opening a portal to a dark dimension or controlling other magical creatures. So he becomes the main obstacle for Harry as he tries to protect the snake.
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.