There could be a group of purist wizards who believe that magical animals should not be put on display for non - wizards to see. They constantly try to disrupt the zoo's operations, causing trouble for Harry. Harry would need to fight for the right of his zoo to exist and educate these purist wizards about the importance of co - existence between wizards, non - wizards, and magical animals.
Well, perhaps Harry could discover a new magical creature in the zoo. This creature could have the power to make other animals communicate in human languages, leading to all sorts of chaos and hilarity as the animals start voicing their opinions and demands.
Maybe an organization that specializes in using the Sharrigan for malicious purposes. They don't want Naruto to expose their secrets, so they send their Sharrigan - users to hunt him down. This organization could be made up of power - hungry individuals who will stop at nothing to achieve their goals.
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.
Maybe a new character could be the main antagonist. A powerful and mysterious being that Luigi stumbles upon during his escape. This character might have some vendetta against the Mario brothers or just be generally evil and wants to stop Luigi's journey for its own malicious reasons.
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.