Who could be the main antagonist in a fanfiction where Naruto is sealed away?

It could be an evil wizard from Negima's world who wants to use Naruto's power for his own dark purposes. This wizard might be aware of Naruto's unique sealing abilities and see it as a way to gain ultimate power over both the ninja and magical worlds.

Perhaps an old enemy has returned, stronger than ever. They might have been plotting during Naruto's sealing and now are ready to take on Naruto and the Hidden Leaf Village. This could be someone like Orochimaru who found a new way to gain power in Naruto's absence.

Maybe it's an ancient evil that was sealed away long before Naruto but managed to break free during his 1000 - year imprisonment. This evil entity now wants to destroy the world and sees Naruto as an obstacle once he is unsealed.

Another option is that a new character introduced in the crossover could be the main character. For instance, in a Naruto - Fullmetal Alchemist crossover, an alchemist from the FMA world could stumble upon Naruto's sealed state and make it their mission to free him, while also getting involved in the politics and power struggles of the Naruto world.

It could be Orochimaru. He might have learned about the Avatar's unique power and wants to use it for his own dark experiments. So he tries to break Naruto's seal to get to the Avatar. He would be a great villain as he's always after powerful things and doesn't care about the consequences.

Another possibility is a character who has been corrupted by the same power that was used to seal Naruto. This character could see Naruto's awakening as a threat to their new - found power or status. For example, if the sealing was related to some ancient and dark chakra, a ninja who has been influenced by this chakra might oppose Naruto.

It could be a new and extremely powerful ninja from a far - off land who has a vendetta against Konoha. His power might be so great that Naruto decides the only way to stop him in the long run is to seal himself away as a sort of trap for when the antagonist gets too close to achieving his goals.

One possible reason could be that his godlike powers were seen as a threat. The village elders or some powerful forces might have feared that Naruto, with his immense strength, could lose control and cause destruction. So, they decided to seal him away to protect the village and the world at large.

There's also the possibility that it was an accident. Some kind of jutsu gone wrong during the sealing process. The person performing the sealing might have been inexperienced or there could have been some unforeseen interference that led to Naruto being sealed away instead of the Nine - Tails being properly sealed within him.

It could be Danzo. He was always power - hungry and had his own ideas about how the village should be run. He might see Naruto as a threat to his plans, so he locks Naruto away as a baby. He wants to control the village without the interference of the jinchuriki's power.