Voldemort could still be an antagonist even if they are trying to change the future. His dark magic and his followers might oppose Harry and Hermione's attempts to rewrite history in a way that doesn't benefit them.
Voldemort's remaining followers could be antagonists. They might see the baby as a new threat or a potential weapon. They could try to kidnap the baby to use its power for their own dark purposes.
Draco Malfoy could be a likely antagonist. He might be jealous of the friendship or whatever relationship is developing between Fred and Hermione and try to cause trouble for them. Another could be some of the more strict professors who don't approve of their unorthodox ways of doing things.
Draco Malfoy could be an antagonist. He might be jealous of Katie Bell's friendship with Harry Potter and try to cause trouble for them. Another potential antagonist could be a Death Eater who is still at large and targets Katie Bell to get to Harry Potter.
The pure - blood supremacists could be antagonists. They might not like the idea of Harry, a half - blood, inheriting Hogwarts and try to undermine his authority.
Maybe there could be a group of rogue wizards who believe that Harry's power to be the master of life is an abomination against the natural order of magic. They would oppose him and try to strip him of his power, seeing themselves as the protectors of the 'true' way of magic. Another potential antagonist could be a power - hungry wizard within the Ministry of Magic who is jealous of Harry's new - found abilities and tries to undermine him at every turn.
It could be a group of dark wizards who have escaped from Azkaban and formed a new dark alliance. They might be seeking revenge on Harry and Hermione for their role in defeating Voldemort.
Often, the goblins can be antagonists. If they have their own motives that conflict with Harry and Hermione's, like protecting a dark secret in the vaults or trying to take advantage of the situation for their own gain. Another possible antagonist could be a Death Eater who has infiltrated Gringotts to either steal something important or to ambush Harry and Hermione.
Often, it could be Death Eaters. They are the main villains in the Harry Potter series, so it makes sense for them to be antagonists in fanfiction where Harry and Hermione need to escape.
There could also be new dark wizards or witches who envy the power and fame that Harry and Hermione have. They might see the child as a way to get to Harry and Hermione, or they might simply want to prove themselves by challenging the family. For example, a dark wizard who has developed a new and dangerous form of dark magic and wants to test it on the child.
In this fanfiction, Voldemort or his remaining followers could be major antagonists. They wouldn't like the idea of Harry having new allies in Percy and the demigod community. From the Percy Jackson side, the Titans or some of the more malevolent gods might not be happy with the alliance either. They could try to break it up or cause trouble for the two boys, seeing them as a potential force against their own plans.