The main antagonist could be the sorcerer who cast the spell on Belle to turn her into a beast. This sorcerer might have his own motives, like wanting to take over the village or the castle where Belle lives. He could be constantly trying to prevent Belle from finding a way to break the curse.
Maybe the main antagonist is a dark spirit that resides in the forest. When Belle becomes a beast, this spirit senses her vulnerability and tries to corrupt her. It tempts her with power and a life without the need to break the curse, but in reality, it wants to use her to spread darkness throughout the land.
It could be a jealous noble from the village. He always envied Belle's beauty and kindness, and when she becomes a beast, he sees it as an opportunity to get rid of her for good. He spreads false rumors about the beast - Belle in the village and tries to turn everyone against her, making her journey to break the curse even more difficult.
The main antagonist could be LeFou. He might be jealous of Gaston's new relationship with Belle and the fact that they are having a baby. So, he could try to create problems between them, like spreading false rumors in the village about Belle's past or Gaston's intentions.
It could be a rogue ninja from a hidden village who has a vendetta against the Green Beast. Maybe this antagonist blames the Green Beast for something that happened in the past.
It could be an envious witch. She was once in love with the prince (now the Beast) but he rejected her. So she cursed him and now she wants to make sure he never finds true love with the Dark Beauty.
It could be another ghost who is jealous of Casper's new human form. This ghost might try to sabotage Casper's attempts to fit into the human world, perhaps by causing chaos around him and making humans afraid of him again.
It could be a character who wants to use Percy - turned - Aku for their own evil purposes. Maybe a dark wizard who thinks that Aku - Percy can be the ultimate weapon to take over the world. This wizard would try to manipulate Percy's new form and powers to do his bidding.
Perhaps a powerful vampire lord who sees Sookie as a threat to his power. Since she was once human and has connections to both worlds, he might fear that she could rally other vampires and humans against him. He could try to eliminate her by any means necessary, leading to a lot of action - packed scenes in the fanfiction.
It could be Sasuke. If Sasuke has turned completely to the dark side and Naruto fails to save him, Naruto might be so frustrated and angry that it starts to make him cold - hearted towards everyone. Sasuke's actions could be the trigger for Naruto's transformation.
The Ministry of Magic could be the main antagonist. They might view Harry's fascination with Dementors as a sign of him turning to the dark side. So they try to stop him from his research and exploration of Dementors, fearing he'll use their powers for evil. After all, Dementors are associated with dark magic in the eyes of the Ministry.
It could be an old and powerful vampire who sees Hermione as a threat to his rule. Since Hermione is a very intelligent witch - now vampire - she might be able to figure out a way to undermine his power.
It could be an old and powerful vampire lord who sees Natsu as a threat to his rule. Since Natsu is a new and different kind of vampire, the lord might want to eliminate him.