It could be a criminal with a vendetta against Beckett. They might abduct the child to get back at her for putting their associates in jail in the past. This antagonist would be calculating and always one step ahead, making it a real challenge for Beckett to rescue the child.
Maybe the main antagonist is a part of an organized crime syndicate. They abduct the child for ransom or to use as leverage in some illegal business deal. The syndicate would have many resources and connections, which would put Beckett in a difficult situation as she tries to rescue the child. They would send their henchmen to try and stop Beckett from getting closer to the truth and to the abducted child.
The main antagonist could be someone who has a personal grudge against the family of the abducted child. This person might be jealous or feel wronged by the family in some way. They think that by abducting the child, they can cause the family the maximum amount of pain. Beckett has to figure out this person's motives and find a way to stop them from harming the child.
It could be a criminal mastermind who has a vendetta against Beckett or someone close to her. This antagonist might see the baby as a way to get back at Beckett, either by using the baby as bait or harming it to cause her pain.
One possible plot could be Beckett's intense search for the abducted child. She might use her detective skills to follow leads, question suspects, and piece together the mystery of the abduction. Another plot could involve the emotional journey of the people related to the abducted child, like the parents, and how Beckett interacts with them during the investigation.
It could be about a father - son duo named Beckett who are suddenly kidnapped. Maybe they are abducted by some criminals who want to use them for ransom. Or perhaps they are abducted by a mysterious group with a hidden agenda related to their family secrets.
Perhaps a deranged fan of Beckett. Maybe Beckett is a well - known figure, like an actor or a writer, and a crazy fan who has an unhealthy obsession abducts them. The fan could be thinking that by doing this, they can get closer to Beckett in some warped way.
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 corrupted Ministry of Magic official. This official wants to use the Muggle world for his own gain, and he sees Kate Beckett as a threat. He teams up with some dark wizards and tries to stop Harry and Kate from uncovering his plans. This would create a complex web of intrigue as Harry and Kate have to fight against not only dark magic but also the corruption within the wizarding world.
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.
A coworker who has a crush on Beckett might be a main character. They could be the one who stays by Beckett's side the most during his recovery, and this could lead to a romantic sub - plot.
Obviously, Beckett would be the main character. But there could also be a hotel concierge who plays an important role. The concierge could be someone who is either very helpful to Beckett, guiding him around the hotel and the local area, or someone who is a bit of a troublemaker, causing difficulties for Beckett during his stay.
A powerful sorcerer who wants to harness the power of the earth and sky for his own selfish gains could be a great antagonist. He might try to corrupt the natural balance to gain unlimited power.