A new, powerful dark wizard could be introduced as the main antagonist. This wizard could be so charismatic and persuasive that he convinces Harry that the traditional ways of the light are wrong and that the power of darkness is the true path to a better wizarding world.
The Ministry of Magic could be antagonists. If they start to oppress those with different views, including Harry, it could drive him towards darkness. Their corrupt and unjust actions might make Harry think that the only way to fight back is by using dark magic.
It could be the Death Eaters who see Harry's new - found mastery of darkness as a threat to their own power and ideology. They might try to either recruit him or destroy him. Voldemort himself could be an antagonist, feeling that Harry is encroaching on his territory as the most powerful dark wizard.
There could be a powerful dark entity that has been awakened by Harry Potter's newfound mastery of fire and darkness. This entity could be an ancient evil that was once sealed away and now wants to use Harry for its own dark purposes or simply destroy him as he is a potential obstacle.
Maybe Harry discovers some dark family secrets. If he finds out that his family has a long - hidden connection to dark wizards, he could be tempted to embrace that part of his heritage. He might start to think that the power associated with dark magic can be used for good, but gradually lose his way as the dark magic corrupts him.
Some of the teachers at U.A. might also be antagonists. They could be secretly working for an organization that wants to control Izuku's powers for their own evil purposes. They would pretend to be helpful while actually setting traps and spreading misinformation about him.
The village elders or certain hidden factions within the village could be antagonists. If Naruto turns dark because of their actions against him, they would naturally be in opposition to him as he seeks revenge or tries to change the village in a way they don't want.
In some fanfictions, a powerful Hollow could be the main antagonist. It might target Orihime because of her special powers and Ichigo has to protect her while also dealing with his new - found feelings for her. The Hollow could be constantly attacking them, trying to separate them.
The obvious choice would be the Prince of Darkness himself. He could be a powerful being with the ability to manipulate shadows and dark magic, constantly trying to draw Percy to the dark side.
It could be the Hidden Leaf Village itself. If the village turns against Naruto through false accusations or unjust treatment, it becomes the force that drives him to the dark side. The elders or some of the more conservative ninja in the village who oppose Naruto's unorthodox ways could be the face of this antagonism.
There could also be new characters introduced in the fanfiction that interact with the main characters from the movie. For example, a local historian who has knowledge about the secrets of Darkness Falls and helps Kyle Walsh in his fight against Matilda Dixon. This kind of new character can add a fresh dynamic to the story and allow for different plot developments.
A powerful wizard who wants to use the 'lady of darkness' powers for his own evil gains could be an antagonist. He might try to manipulate her into doing his bidding.