Another possibility is a high - ranking Imperial officer who is jealous of the OC's abilities or connections. This officer would try to sabotage the OC's missions and get them in trouble with the higher - ups. They might spread false rumors about the OC being a Rebel sympathizer just to get rid of the competition.
It could be a Rebel spy within the Empire. This spy would be constantly trying to undermine the OC's progress and expose their true intentions. The spy might see the OC as a threat to the Rebellion's plans, especially if the OC is rising in the ranks quickly.
The plot could involve the original character (OC) being attracted to the Empire's power and order. Maybe the OC is from a chaotic planet and sees the Empire as a way to bring stability. They might start as a low - ranking recruit, facing challenges from other Imperials who don't trust outsiders. But as they prove their loyalty and skills, they rise in the ranks and get involved in important missions, like hunting down Rebel spies.
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.
Another possibility is a group of Imperial scientists who are conducting unethical experiments with the Force. They see Luke as a prime specimen and try to control him. They would be the main antagonists as they try to use Luke for their own immoral purposes rather than allowing him to pursue his own agenda within the Empire.
A group of ninja who oppose the new order of the Naruto Empire could be the main antagonists. They might believe that the empire has become too controlling or has strayed from the original values of the ninja world. They could use guerrilla tactics and create internal unrest within the empire to try and bring it down.
It could be an old enemy of the Hunters. Maybe a powerful monster that has a grudge against them for past defeats. For instance, a giant who has been waiting for the right time to strike back at the Hunters.
It could be a rogue wizard who has infiltrated SHIELD and is using their technology for dark magic purposes. This wizard might be jealous of Harry's new position and try to undermine him.
The main antagonist could be Madara. Even though Obito was once following Madara's plan, with Naruto joining Obito, Madara might see this as a threat to his grand scheme. He could try to manipulate events to drive a wedge between Naruto and Obito or directly attack them to stop their new alliance from achieving their goals.
It could be an ordinary human on Earth who rises to become a hero against the Empire. Maybe a scientist who figures out a way to counter the Empire's force powers.
In a 'Harry Potter joins Empire Star Wars' fanfic, Harry might find himself in the middle of the Galactic Empire. He could use his invisibility cloak to sneak around Imperial bases. His wand could be seen as a strange new weapon. And perhaps he would team up with some rebels, sharing his knowledge of magic to help them in their fight against the Empire.
If Luke joined the Empire in a fanfic, it would be a complete deviation from his original path. He might use his Force abilities to rise quickly within the Empire's ranks. However, his friends like Han and Leia would likely become his enemies, leading to intense and tragic confrontations.