A new dark wizard or witch who has a vendetta against Parvati's family. They could be using Harry's mind - controlling spells to make him do their dirty work of kidnapping her.
It could be Voldemort or one of his Death Eaters. They might be using Harry as a pawn to get to Parvati for some reason, like she has a special power or knowledge that they want.
Well, it could be that Harry is under the influence of a dark spell which makes him kidnap Parvati. Then perhaps his friends, like Hermione and Ron, have to find a way to break the spell and rescue Parvati while also trying to understand why Harry did this in the first place.
It could be Orochimaru. He has always been interested in Sasuke and might kidnap Sakura to use her as bait to get Sasuke back under his control. He could be after Sasuke's power and think that by using Sakura, he can manipulate Sasuke easily.
It could be an archaeologist who has been corrupted by the power of an ancient artifact related to the mummy. This archaeologist wants to use the mummy's power for his own gain, like becoming immortal or ruling the world. He would be constantly trying to stop Harry from helping the mummy.
It could be a powerful dark wizard who has also gained some supervillain - like powers. This antagonist might be jealous of Harry's newfound superhero status and wants to destroy him.
It could be a jealous wizard who sees the relationship between Harry and the Lamia as a threat to his own power. He might try to disrupt the harmony in the circus and drive the Lamia away.
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.
It could be a rogue wizard who has infiltrated SHIELD and is using their technology for dark magic purposes. This wizard might be jealous of Harry's new position and try to undermine him.
It could be a jealous wizard who feels that Harry and his sister are getting too much attention in the wizarding world. This wizard might try to undermine their achievements and even put them in danger.
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.