It could be a high - ranking Imperial officer. The Empire might view the rogue knight as a dangerous rebel, even if the knight has no connection to the Rebel Alliance. This officer would use all the resources at the Empire's disposal to hunt down the rogue knight and bring them to justice, according to the Empire's laws.
The main antagonist could be a powerful Sith Lord. This Sith Lord might see the rogue knight as a threat to their plans for galactic domination. Since the rogue knight is operating outside the normal Jedi - Sith conflict, the Sith Lord would want to eliminate this unpredictable element.
Perhaps a dark Jedi. A dark Jedi who has formed their own cult could see the rogue knight as a rival. The dark Jedi's teachings might be different from the traditional Sith, and they would want to prove that their way is superior by defeating the rogue knight. They might also be after the secrets or powers that the rogue knight possess.
Well, the rogue knight could be a Jedi who has broken away from the Jedi Order. Maybe they've seen the flaws in the Order's ways and decided to fight the Sith on their own terms. They might travel the galaxy, encountering various species and getting involved in battles that the official Jedi or Sith may not be aware of.
It could be a rogue Sith Lord who has broken away from the traditional Sith teachings and is causing destruction in his own way. He might not be interested in the age - old battle between the Sith and the Jedi but just wants to spread chaos for his own reasons.
Perhaps an ancient evil force that Darth Vectivus accidentally awakens. This force is much more powerful than him and starts to control him, making him do things he doesn't really want to. Now Darth Vectivus has to find a way to break free from this new master while also dealing with the rest of the galaxy that sees him as the bad guy.
Darth Vader could be a great main antagonist. His power in the Force and his imposing presence would be a huge threat to The Flash. He could try to capture The Flash to study his speed abilities for the Empire.
Adventure is also a big theme. The Rogue Knights are known for their daring exploits, so fanfictions often involve them going on quests, exploring new planets, or uncovering ancient secrets. It could be something like finding a lost Jedi holocron or a powerful Sith artifact.
A scruffy - looking smuggler like Han Solo would be a great main character. He has the charm, the piloting skills, and the roguish attitude that would fit well into this kind of story.
An ex - Imperial officer could be the main character. After seeing the atrocities committed by the Empire, he goes rogue. He has inside knowledge of the Empire's military strategies and uses this to help the Rebel Alliance. However, he also has to deal with the fact that many in the Alliance don't trust him at first because of his past with the Empire. He has to prove himself and find his place in the fight against the Empire.
The main antagonist could be a rogue Imperial officer who has his own agenda. He's tired of following the Emperor's orders and decides to create his own power base. He targets Delta Squad because they keep getting in the way of his plans to steal valuable resources and technology from various planets.
Lex Luthor could be a great antagonist. He might see Rogue Lemon as a new variable in his plans to defeat Superman. He could try to manipulate Rogue Lemon into turning against Superman or use her powers for his own evil purposes.
The Hidden Leaf Village itself could be the main antagonist. If they turn on Naruto for no good reason, they become the force he has to fight against as a rogue ninja.