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.
A dark entity from another dimension could be the main antagonist. It could have invaded Elsword's world and latched onto him, slowly corrupting his soul. This entity's goal could be to use Elsword to open a gateway to other worlds and spread darkness throughout all of them.
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.
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.
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 plotline could involve a betrayal within the group that causes Elsword to turn dark. For example, if he discovers that one of his closest allies has been secretly working against him all along, the shock and anger could drive him to the dark side. He might then start to seek revenge on not only the betrayer but also those he perceives as being part of the conspiracy.
A powerful and corrupt Pokemon trainer could be the antagonist. This trainer is jealous of the boy's unique transformation into a Pokemon. He wants to use the boy's powers for his own gain, like winning all the Pokemon battles and championships. So, he hunts the boy down, causing all sorts of trouble for him during his journey as a Pokemon.
Maybe Team Rocket could be the main antagonist. They might see Ash's transformation as an opportunity to finally capture Pikachu. Since Ash is in a more vulnerable state as a girl, they could try to use different tactics to take advantage of the situation.
Perhaps a jealous trainer. This trainer has always been envious of Ash's success and now that Ash is in a vulnerable state as a Zorua, the trainer sees an opportunity to get rid of him for good. The trainer could try to use his own Pokémon to capture Ash or even harm him, making the trainer a formidable antagonist in the story.
Perhaps an evil mage who actually intended for the transformation to go wrong on purpose. They might want to use Sans' new female form to cause chaos in the Underground. They could be trying to manipulate Sans into doing bad things or using her to get to other important characters.