Another possibility is that the main antagonist could be Death itself, personified. If Harry is becoming death - like, Death might see him as a threat or as someone who is trying to usurp its role. So, Death could be constantly trying to test Harry, or even try to eliminate him to maintain its own dominion over the cycle of life and death.
Perhaps a new character, a dark wizard who wants to use Harry's new connection to death for his own evil purposes. This wizard could be jealous of Harry's power and try to manipulate or control him. He might see Harry as a tool to gain more power over life and death, and would stop at nothing to achieve his goals.
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.
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.
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.
Another possibility is that some members of the wizarding government who oppose the idea of Harry being in Malfoy's care could be the antagonists. They might try to undermine the situation, create false accusations against the Malfoys, or even try to kidnap Harry to put him in what they consider a more 'appropriate' place.
It could be a powerful wizard who sees Harry's transformation as an abomination and wants to destroy him. This wizard might believe that Nagas are dark creatures and that Harry, by becoming one, has crossed a line into the dark side of magic.
There could be a group of pure - blood wizards who believe that dragons are beneath them and that Harry is tainting the wizarding world by associating with them. They would try to stop Harry from his dragon - related activities, and they might use underhanded tactics like casting spells on him or trying to get him expelled from Hogwarts.
It could be a dark wizard who wants to control the dragons for his own evil purposes. He might see Harry as a threat because Harry is forming a bond with the dragons and teaching them to resist dark magic.