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.
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.
There could be a new dark wizard or witch emerging. This character could be jealous of Harry and Hermione's influence and see the baby as a way to get to them. They might try to harm the baby or turn it against Harry and Hermione, creating a lot of conflict and danger in the story.
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.
Wart could also be an antagonist. He might try to take over the Mushroom Kingdom where Baby Mario and Baby Peach live. He could send his minions to stop the two from getting help or finding special items that can defeat him.
Cell could also be an antagonist. He has an interest in powerful Saiyan genetics. The baby, being related to Goku, would be of great interest to him. He might try to absorb the baby to gain even more power, creating a big conflict for the heroes to deal with.
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.
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.
One potential antagonist could be some pure - blood supremacists who don't like the idea of Harry Potter having a half - blood or Muggle - born daughter. They might try to cause trouble for Lemon at Hogwarts or in the wizarding world in general. Another could be a dark wizard who sees Lemon as a threat for some reason, perhaps because of her special abilities or her connection to Harry. They would try to harm her to get to Harry.