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.
There could be a group of antagonists. For instance, some of the Death Eaters from Harry Potter's world could team up with Hydra agents from the Avengers' world. They would combine their dark magic and evil technology to cause mayhem. This would force Harry Potter and the Avengers to put aside their differences in their ways of doing things and work together to stop this combined force. The Death Eaters' knowledge of dark spells and Hydra's devious plans would make them a formidable foe for Harry and the Avengers.
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 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.
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.
A powerful magical creature that has a grudge against both Harry Potter and Ace. It might see Ace as an intruder in its territory in the Harry Potter universe and Harry as someone who is interfering. So it tries to stop Harry from saving Ace and becomes the main antagonist.
There could be a new character created specifically for the fanfic. A dark wizard who has been observing the wizarding world from the shadows and sees Hisoka's arrival as an opportunity. This new antagonist could try to use Hisoka to achieve his own evil goals, like opening a portal to a dark dimension or stealing all the magic in the world. When Hisoka realizes he's being used, he would turn against this new antagonist, and Harry Potter and his friends would join him in the fight.
Voldemort could be a likely main antagonist. He's the most powerful dark wizard in the Harry Potter series. He might see the Avengers as a threat to his plans of total domination, not understanding their world but still seeing their power. So, he could use his dark magic to torture them, trying to break their will before eliminating them.
Well, perhaps Harry uses his magic skills like the Patronus charm to fend off some dark force that is attacking the Avengers. He could also use his knowledge of potions to heal any injured Avengers.
If Harry Potter saves the Avengers in fanfiction, he would bring a whole new dimension to their battles. His knowledge of the magical world could be used to outwit enemies. He could create magical barriers to protect the Avengers' base or use the invisibility cloak to sneak up on foes. And his Patronus could be a great weapon against dark forces that the Avengers face. It would be a unique crossover where magic and superhero powers combine.