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.
It could be Darth Sidious. He is always looking for powerful allies. If he senses Ahsoka's potential and her growing disillusionment with the Jedi, he might manipulate her into joining the Dark Side. His cunning and power would make him a formidable antagonist in this scenario.
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 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.
Perhaps it's one of Percy's former friends who has turned against him. This friend could be jealous of Percy's powers or has been influenced by some dark force. Their actions and taunts could be what drives Percy to become dark, making them the main antagonist in the story.
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.