The main antagonist could be a dark spirit that has been trapped in the forest around the Beast's castle for centuries. It feeds on negative emotions and tries to drive the Dark Beauty and the Beast apart because their love and hope would weaken its power.
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.
Perhaps a power - hungry nobleman. He hears about the Beast's castle full of treasures and wants to take over. He sees the Dark Beauty as a way to get to the Beast and tries to stop their relationship from developing so he can have the castle for himself.
Perhaps a group of dark side cultists could be the main antagonists. They idolize the power of the dark side and see Ahsoka as a prime candidate to join their ranks. When she refuses at first, they start to hunt her down, but in the process, they unknowingly push her closer to the dark side, creating a complex dynamic where they are both the pursuers and the inadvertent cause of her fall to the dark side.
The Green Goblin could be a great main antagonist in a dark fanfiction. He could be even more deranged and have a more elaborate plan to destroy Spider - Man, not just physically but also mentally by targeting his relationships.
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.
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.
It could be an unknown ninja who has been secretly studying dark energon for years. He wants to use it to take over the ninja world, believing that the power of dark energon will make him invincible.
Well, obviously Dark Gaia itself could be the main antagonist. It has the power to cause chaos and destruction, and Sonic would have to stop it from wreaking havoc on the world.
It could be a powerful sorcerer. In the Dark Skies world, a sorcerer with dark magic could oppose Jesse. This sorcerer might be after the same magical artifacts as Jesse or simply wants to rule the Dark Skies with an iron fist.
It could be a rogue Dark Jedi who has escaped from his own Order and sees the Naruto world as a new place to spread chaos. He might be seeking powerful chakra sources to fuel his Dark Side powers.
It could be Sasuke. If Sasuke continues to pursue his own vendetta in a more extreme way and Naruto's attempts to bring him back fail, Sasuke could become the main antagonist. Their rivalry could turn into a full - blown battle for power and ideology, with Naruto being pushed to his limits and potentially turning dark in the process of trying to stop Sasuke.
It could be an ancient dark wizard who has been manipulating events from the shadows. This wizard might see Elsword as a powerful tool and has been slowly influencing him to turn dark. The wizard could be using dark magic to cloud Elsword's mind and make him more susceptible to negative emotions.