A powerful magical creature that has been wronged by humans in the past and blames the Potter family for not doing enough to protect its kind. So it decides to take revenge on Harry's child. It uses its unique magical abilities to create all sorts of obstacles for the child, from causing problems in their classes at Hogwarts to more dangerous situations in the Forbidden Forest.
Perhaps a group of pure - blood supremacists who didn't like that Harry, a half - blood, became so powerful and respected. They see Harry's child, who is also a symbol of the new, more inclusive wizarding world, as a threat. They try to undermine the child's confidence and attempts to make positive changes in the wizarding community.
It could be a new dark wizard who is jealous of Harry Potter's fame and success. This dark wizard targets Harry's child, thinking that by causing trouble for the child, they can somehow diminish Harry's reputation in the wizarding world. He might try to frame the child for various dark magic incidents around Hogwarts.
Voldemort could still be the main antagonist. Even if Harry is a 'child of magic' in a different way, Voldemort would still see him as a threat to his power and his vision of a pure - blood dominated wizarding world.
It could be a distant relative of Voldemort who has been biding their time in the shadows. They might have a vendetta against the Potter family for what happened to Voldemort. This relative could be secretly gathering followers and planning to strike at the second child when they least expect it.
The main antagonist could be a powerful necromancer who combines the dark arts of Skyrim with some elements of Voldemort - like magic. He could be raising an army of the undead and causing chaos across Skyrim, which Harry has to stop.
It could be a new dark wizard who has a vendetta against Harry. This wizard might use a spell to stop Harry from eating, thinking it will weaken him before a final showdown. He could be someone who was once a follower of Voldemort and now wants to take his place and prove himself by getting rid of Harry.
Maybe an ancient evil spirit that was once bound but is now trying to break free by using the summoner as a conduit. This spirit could be manipulating events from the shadows, causing chaos in the wizarding world to draw the summoner out.
It could be a Frost - Witch. She might be jealous of Harry's connection to magic from Hogwarts and wants to use his powers for her own evil plans in the frost - covered land.
A new dark wizard or witch could be introduced as the antagonist. This character could be even more powerful and cruel than Voldemort. They could have a special power that allows them to break Harry's spirit, perhaps by constantly torturing his mind or by causing harm to those he loves in ways that are even more insidious than Voldemort's methods.
Perhaps a long - lost relative of Voldemort who has been in hiding all these years. This relative has discovered a way to gain power through Harry's kid. They might start by infiltrating Hogwarts under a false identity and slowly manipulate events to get closer to the kid. Their ultimate goal could be to sacrifice the kid in a dark ritual to bring back the dark days of Voldemort's reign.
A new dark wizard could be the main antagonist. Maybe someone who was inspired by Voldemort's ideology but has new and more devious ways of causing chaos in the wizarding world. This antagonist could target Harry Potter's son to prove himself as a more powerful and feared dark wizard.
Perhaps an ancient spirit that has been awakened by the imbalance in the life - death cycle due to Harry becoming the Master of Death. This spirit sees Harry as the cause of the chaos and tries to destroy him to restore the balance in a way that is harmful to the wizarding world.