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.
One idea could be exploring Darth Vectivus's early life and how he first got involved in the dark side. Maybe he had a difficult childhood that led him down this path.
The main antagonist could be a rogue Jedi. This Jedi has seen the dark side of the Clone Wars and decides to take matters into their own hands, often clashing with Darth Vader's plans. He might be trying to stop Vader from causing more destruction during the war.
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.
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.
In Star Wars fanfiction, Darth Imperius is often a unique Sith character created by fans. He might be depicted as having a complex backstory, perhaps rising through the ranks of the Sith in a different way compared to the well - known Sith Lords in the official Star Wars canon. His powers and motives are usually crafted by the fanfic authors to fit within the Star Wars universe's dark side themes.
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.
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.
Maybe some of the Jedi who cross paths with Darth Maul, like Qui - Gon Jinn. Their interactions are important in the Star Wars universe. Also, there could be other Sith or dark side characters who are part of Maul's story, perhaps his own apprentices if he has any at some point in the story.
Darth Maul could be a great antagonist. His double - bladed lightsaber would be a formidable weapon against Naruto. Maul's acrobatic fighting style and his Sith powers would pose a huge challenge to Naruto's ninja skills and new - found Jedi powers.
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.