Maybe a powerful sorceress who was once banished from Cintra and now wants revenge. She has amassed a dark army and is using dark magic to try and take over the land. Harry Potter has to figure out her weaknesses and stop her before it's too late.
It could be a dark wizard who has his eyes on Cintra's powerful magical artifacts. He wants to use them to gain more power and is willing to destroy Cintra to get them.
There could be a group of dark mages who form a council. They believe that Cintra's magic should be harnessed for their own evil purposes. They start by causing chaos in the land, cursing the crops and spreading fear among the people. Harry Potter has to face this council and break their hold on Cintra.
One possible plot could be that Harry, through some magical accident or powerful spell, is transported to the world of Cintra. There, he discovers the danger it's in. Using his magical skills like the Patronus charm to ward off dark creatures threatening Cintra, and his knowledge of spells to help in battles. He might also form alliances with the local warriors, teaching them some basic magic for self - defense.
It could be a rogue Death Eater who has escaped from Azkaban. This Death Eater might be desperate to regain his power and believes that unicorn blood will give him the strength he needs. So he sets out to capture a unicorn in the Forbidden Forest, which is where Harry comes in to stop him.
It could be a dark wizard who has a vendetta against goblins. This wizard might see goblins as a source of power or wealth and wants to enslave them. So when Harry tries to save the goblin, this dark wizard becomes the main obstacle.
It could be some dark wizards who want to use the centaur's parts for potions. They might be following some ancient, dark magic recipe that requires centaur hair or hooves. So they hunt down the centaur, but Harry gets in their way.
It could be an ancient dark magic artifact. The artifact has a hold on Bellatrix, and in a way, it's the 'enemy' that Harry has to defeat to save her. The artifact might be trying to use Bellatrix to carry out some dark plan, like opening a portal to a world full of dark creatures. Harry has to figure out how to break the connection between Bellatrix and the artifact while also fending off the artifact's attacks.
A group of dark creatures like Dementors could be the antagonists. They might be after the young goblin because goblins have a unique life force that can sustain Dementors in a way other prey cannot. Harry Potter would have to face his own fears while using powerful Patronus magic to drive the Dementors away and save the goblin.
Perhaps a dark wizard from the Harry Potter universe who has heard about the Cullens' unique vampire abilities and wants to harness that power for himself. He could be a follower of Voldemort who escaped the final battle and is now looking for new ways to gain power. So he targets the Cullens, thinking their powers can help him rule both the wizarding and vampire worlds, and Harry has to stop him.
Maybe a new character could be created as the main antagonist. A dark wizard who has infiltrated the Capitol and is using the Hunger Games as a means to create chaos and destruction. He could be more powerful than Voldemort in some ways and would be a great challenge for Harry and Katniss to overcome.
It could be a powerful dark wizard who has somehow made his way into the Avengers' universe. Maybe a wizard who was banished from the wizarding world in Harry's universe and has now joined forces with some of the Avengers' villains.
Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.