The main antagonist in a 'Criminal Luke Skywalker' fanfic could be a powerful crime syndicate. They might see Luke as a competitor in their illegal activities. For example, if Luke starts stealing from their smuggling operations or interfering with their black - market deals. They would send their enforcers and assassins after him. Also, there could be a rogue Jedi who has turned to the dark side before Luke. This rogue Jedi might see Luke as a rival for the leadership of the criminal underworld among Force - users and try to eliminate him to assert his own dominance.
One possibility for the main antagonist is a group of bounty hunters. Since Luke is now a criminal, they would be after the bounty on his head. They could be led by a particularly ruthless and skilled bounty hunter who has a personal vendetta against Luke. Maybe Luke once thwarted this bounty hunter's plans in the past, and now he sees this as a chance for revenge. Additionally, the New Republic could also act as an antagonist in a sense. They would see Luke as a threat to the peace and stability they are trying to build and would send their own forces to capture him.
A new alien race with a natural inclination towards the dark side could be the main antagonist. They see the Jedi as a threat to their way of life, which is based on conquest and domination. Luke has to figure out a way to either defeat them or find a way to coexist while still upholding the Jedi values.
One possible plot could be that Luke Skywalker turns to the dark side and starts committing crimes against the New Republic in order to gain more power for himself. He might start stealing valuable artifacts or information that could give him an edge. Another plot could be that he is framed for crimes he didn't commit and has to go on the run, trying to clear his name while also being hunted by the authorities. A third plot could involve Luke being under the influence of an evil force or entity that makes him act in a criminal way, and his friends have to find a way to break the influence and save him.
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.
Roman Torchwick would be an obvious choice. He already has a criminal background in the RWBY series. He could return with a new and more devious plan, perhaps teaming up with other villains to cause chaos.
It could be a detective who is hell - bent on capturing Ash. This detective might have a personal vendetta or just be really dedicated to bringing criminals to justice.
Darth Vader could be an unexpected ally in a time travel scenario. If Luke travels to a time when Vader was still Anakin Skywalker and on the verge of turning to the dark side, Luke could try to prevent his fall. In the process, Anakin might become an ally, using his skills as a powerful Jedi to help Luke in whatever mission he has in that time period.
One great Luke Skywalker fanfic to start with could be 'The Life and Times of Luke Skywalker' as it delves deep into his character, exploring his journey not just as a Jedi but also as a person. It gives a very in - depth look at his relationships, his growth, and his struggles, which is really engaging for any Star Wars fan.
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.
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 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.