The Force itself could be an antagonist in a way. Even though the Force has no will in the traditional sense, Luke's turn to the dark side could be seen as going against the natural balance of the Force. So, the Force could subtly oppose him, causing unexpected challenges and difficulties as he tries to fully join the dark side. For example, it could make it harder for him to access certain dark side powers or make him face internal conflicts due to his previous connection to the light side.
It could be the remaining Jedi. Since Luke is turning to the dark side, the Jedi who still uphold the light side values would be against him. They would see him as a threat to the balance they are trying to maintain and would try to stop him from fully embracing the dark side.
It could be the remaining Jedi. Since Luke has joined the dark side, the Jedi who still uphold the light side would be his natural antagonists. They would see him as a threat to all they stand for and would try to stop him from spreading the influence of the dark side. For example, Leia, if she still adheres to the Jedi teachings, would be in direct opposition to Luke.
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.
Perhaps a powerful dark side cult leader. They could manipulate Ezra into believing that their way is the right one and that the dark side is a misunderstood power. The cult leader might use Ezra's own insecurities and desires to draw him into 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 the remaining Jedi. Since Rey is turning, the Jedi who still uphold the light side would see her as a threat. They would try to stop her from causing more harm with her newfound dark side powers.
It could be Dumbledore. In a twisted take on the story, if Harry joins the dark side, Dumbledore would be the one trying to stop him. Dumbledore has always been the guiding force for the good side, and Harry's defection would be a huge blow. Dumbledore would then have to face the difficult decision of either trying to save Harry or stopping him by any means necessary, making him the antagonist from Harry's new 'dark' perspective.
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 the headmaster of the imperial academy. They would be in charge of making sure all the students adhere to the Empire's values and teachings. They might see Luke as a potential threat to their regime within the academy and constantly try to undermine him or turn him into a loyal imperial subject.
In the fanfiction, Luke could be influenced by his family ties to the dark side. If he discovers some hidden connection to Vader or Palpatine that he didn't know before, it might make him question his place on the light side. He could think that joining the dark side is a way to understand his true heritage better and gain more power to protect those he loves in a different way than the Jedi would allow.
Dumbledore could be seen as the main antagonist from Harry's new perspective on the dark side. Dumbledore represents all that is good in the wizarding world, and if Harry has turned to the dark, Dumbledore would be the one trying to stop him, to bring him back to the light. So in this context, Dumbledore becomes the opposition that Harry has to face.