Maybe it's a powerful Muggle criminal who has somehow learned about the wizarding world and wants to use its secrets for his own gain. He poses a threat not only to Muggles but also to wizards, and Harry, with his gun, has to stop him from causing chaos. This antagonist could have a large network of followers and access to advanced Muggle weaponry, making him a formidable foe.
It could be a dark wizard who has found a way to nullify magic in certain areas. This antagonist forces Harry to rely on the gun as his magic is useless against him in those zones. He might be a former Death Eater who has new, more dangerous powers and a vendetta against Harry for his role in the downfall of Voldemort.
In a post - war fanfic, Harry could be living in a more integrated wizard - Muggle society. There are still remnants of dark forces, but now the wizarding world is more open to Muggle technology. Harry, being a symbol of protection, decides to carry a gun as an additional means of safeguarding the peace. He trains with Aurors and Muggles alike to master the use of the gun, and along the way, he faces challenges from traditional wizards who oppose this new direction.
It could be a rogue wizard who has also acquired Muggle weapons and is using them to cause chaos. This wizard might be jealous of Harry's fame and wants to use the gun to prove that he's more powerful.
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.