Who could be the main antagonist in a 'naruto code geass crossover fanfiction'?

Madara Uchiha could be a great main villain in such a crossover. His already powerful ninja abilities combined with the potential of getting a Geass power would make him extremely dangerous. He could use Geass to control other ninjas and make them fight for his cause, like creating an army of mind - controlled shinobi to take over the world.

It could be a new character who is a mix of Naruto and Lelouch. He has Naruto's determination and Lelouch's intelligence. This character could be the center of the harem, with his unique personality attracting female characters from both universes.

Another possibility is that the main antagonist could be an organization within the Britannian Empire that is secretly pulling the strings. They might see Lelouch and Leila as a threat to their hidden agendas. This organization could be using underhanded tactics like assassination attempts, spreading false rumors, and creating internal conflicts within Lelouch and Leila's alliance to try and bring them down.

A new character could be created. This character could be a powerful slug - human hybrid who has been corrupted by the dark side of both worlds' powers. He wants to rule over both Naruto's world and Slugterra, believing that the two worlds should be united under his rule. He has the ability to use both ninja techniques and slug powers, making him a very difficult opponent to defeat.

Perhaps an ancient and evil ninja from a long - lost clan that was sealed away behind the gate. He resents the modern ninja world and wants to destroy it. He has unique and powerful jutsus that Naruto and his friends have never seen before.

It could be a powerful dark spirit from Kagome's world that somehow manages to cross over to the Naruto universe and starts causing chaos. This spirit might be immune to normal ninja attacks and require Kagome's spiritual powers to be defeated.

Maybe another kitsune who has gone bad. In the fanfiction world, there could be a rogue kitsune who sees Naruto as a threat. This antagonist kitsune might be jealous of Naruto's power or his connection to the human world. They could try to undermine Naruto's attempts to protect the village or his friends.

Madara Uchiha would also make a great antagonist. Imagine if he was resurrected and saw Naruto with Samehada as a threat to his plans of creating a new world order. He would use his Sharingan and Rinnegan powers to try and take Samehada from Naruto, believing that with it, he could easily defeat all who oppose him and reshape the ninja world according to his will.

Another possibility is a corrupted dragon spirit that has been awakened due to the mixing of the two worlds. This spirit could be spreading chaos and trying to destroy both the ninja world and the dragon world. Naruto would have to figure out a way to stop it, as it has powers that are beyond what he has faced before.

Perhaps an ancient evil that has been sealed away in the empire for centuries but is released due to the chaos of the crossover. This entity could be more powerful than any foe the Naruto characters have faced before, forcing them to find new ways to fight.