It could be an existing villain from the original story who sees Adrien's potential and manipulates him into turning to the dark side. For example, a powerful sorcerer or a mastermind criminal. They could use Adrien's weaknesses, like his desire for power or his need to prove himself, against him.
Another possibility is that Adrien himself could be his own antagonist in a sense. His inner demons, his self - doubt, and his negative emotions could be personified as an internal force that drives him to the dark side. So, instead of an external foe, it's his own psychological struggles that act as the main antagonist, pushing him to make choices that lead him into the darkness.
Perhaps a new character created specifically for the fanfic. This character could be a dark entity that has been observing Adrien from afar. It could disguise itself as a friend or a mentor at first, but then slowly reveal its true intentions and lead Adrien astray. The new character could have a personal vendetta against the forces of good and sees Adrien as a key weapon in its fight.
It could be Hawk Moth. He might finally succeed in corrupting Adrien. Hawk Moth could use Adrien's inner doubts and fears to his advantage. For instance, if Adrien is worried about losing his loved ones, Hawk Moth could promise him a way to keep them safe if he joins the dark side.
Palpatine could still be the main antagonist even if Luke turns to the dark side. He might try to use Luke as a more powerful weapon against any remaining Jedi or the Rebel Alliance. Palpatine would see Luke's newfound dark side power as a means to further his own control over the galaxy.
It could be a new Sith Lord created specifically for the fanfiction. This new character could be more powerful and devious than any before. He could be the one who lures Luke to the Dark Side and then becomes his enemy as they compete for power within the Dark Side hierarchy.
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.
A Sith lord who has been secretly observing Ashoka could be the main antagonist. This Sith lord might see Ashoka's potential and want to turn her to the dark side to use her power for their own evil plans.
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.
It could be Dumbledore. If Harry turns to the Dark Side, Dumbledore, who has always been a guiding figure for Harry, would be the main one to oppose him. Dumbledore represents the light and everything Harry is now against. Dumbledore would try to stop Harry from causing chaos and destruction in the wizarding world.
Another possibility is that an akumatized character becomes the main antagonist in this fanfic. For instance, a jealous classmate of Adrien's who gets akumatized. Their power could be to drain the energy or mobility of others, and they choose to target Adrien first. This would create a more personal and complex conflict within the story, as the akumatized character has a pre - existing relationship with Adrien.
Maybe the main antagonist is an evil scientist. The scientist has been studying Pokémon transformations and when he sees Ash turn into Pikachu, he wants to experiment on him. He chases Ash around, trying to capture him with all kinds of high - tech devices. This forces Ash to use his Pikachu form's abilities and his human knowledge to outwit the scientist.