The Emperor or some other powerful Sith lords would likely be involved. They would see Luke as a great prize to turn to the dark side and would use all their cunning and power to manipulate him. Yoda's spirit could also be present, trying to guide Luke from beyond the grave but perhaps being unable to prevent his fall. Han Solo might also play a role, either trying to rescue Luke or being a victim of Luke's new dark side - influenced actions.
There could be new characters created specifically for the fanfiction. For example, a dark side apprentice who befriends Luke and gradually leads him astray. Or an ancient Jedi who has fallen to the dark side long ago and now resurfaces to tempt Luke with forbidden knowledge. Old characters like Chewbacca might also be there, perhaps being horrified by Luke's transformation and trying to do something about it.
Obviously, Rey is the central character as the story is about her fall to the dark side. Kylo Ren might also be a key figure. He could be either a catalyst for her fall, perhaps by tempting her with the power of the dark side, or he could be someone who tries to bring her back later on. Finn and Poe are also likely to be involved. They would probably be the ones trying to save Rey from the clutches of the dark side.
In some fanfictions, Luke might fall to the dark side after being betrayed by his closest friends. For example, if he discovers that they have been hiding important information from him that he thinks could have saved lives. This sense of betrayal could push him towards the dark side as he feels abandoned by the light. Also, a powerful dark side user could use mind control techniques on Luke, slowly turning him over time.
Voldemort would surely be involved. He might initially be suspicious of Harry's sudden change but then see it as an opportunity to use Harry's power for his own dark ends. He could try to mentor Harry in the ways of dark magic.
In some fanfictions, 'dark side Luke' might be paired with Palpatine. Palpatine, as a master manipulator, could have a role in Luke's fall to the dark side, and their partnership would be full of political intrigue and dark Force - related activities. Palpatine could use Luke as a tool to further his own evil plans, while Luke might be seeking more power from Palpatine.
Obviously Rey herself is the central character. Kylo Ren often plays a role too. In some fanfictions, he might be the one who either tries to stop her from fully turning to the dark side because he has some sort of connection with her, or he might be the one who encourages her to embrace the dark side to be with him.
Luke's friends from the light side could also be important. Characters like Leia, Han Solo, or Chewbacca. Their reactions to Luke's turn and their attempts to bring him back would be significant aspects of the story. There could also be new characters created specifically for this novel, perhaps some Jedi who were thought to be dead but are actually in hiding and oppose Luke's new path.
Obviously, Shawn and Gus would be involved. They are the main duo in 'Psych'. Juliet and Lassiter might also be part of it, either helping Shawn or being skeptical of his methods.
In addition to the above, other characters from the wizarding world could be involved. For example, the Order of the Phoenix members. They might be suspicious of Harry at first but then come to his aid. And there could be some new characters created specifically for the fanfiction. Maybe a mysterious old wizard who has knowledge about the Dark Mark that no one else does and helps Harry on his journey to rid himself of it or at least understand it better.
The Trix might also be involved. Since they are on the dark side, they could either be the ones who tempt Bloom to join them or be rivals who don't want Bloom encroaching on their territory. Also, the headmistress of Alfea College could be in it, trying to figure out what to do about Bloom's situation and how to protect the other students from the possible threat that a dark - side Bloom could pose.
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.