Who could be the main antagonist in 'Naruto Dies and Becomes a Soul Reaper' fanfiction?

One possible plot could be that Naruto, after dying heroically in his own world, is sent to the Soul Society. There, he discovers his new spiritual powers as a Soul Reaper. He might struggle at first to adapt to the new rules and hierarchy. He could meet old friends in spirit form and together they train to become strong Soul Reapers. Naruto's determination and unique ninja skills might give him an edge in this new afterlife, and he could end up facing some great evil in the Soul Society, using his newfound powers in unexpected ways.

Perhaps it's a rogue Shinigami who doesn't want Naruto to figure out how to control his new form. This Shinigami might fear that Naruto could disrupt the balance between Hollows and Shinigami if he becomes too powerful in his hollow state. So, he tries to stop Naruto at every turn, creating a lot of conflict in the fanfiction.

It could be a new character created just for the fanfiction. Maybe a rogue ninja who has mastered a dark and powerful jutsu that Naruto has no defense against. This antagonist could be seeking revenge on Konoha for some past wrong and sees Naruto as the biggest threat to his plans.

It could be someone who was secretly behind Naruto's death in the first place. Maybe a rogue ninja who has a vendetta against the Hidden Leaf and saw Naruto as a major obstacle.

It could be an ancient evil that was sealed away long ago but has been released due to Naruto's death. Maybe Naruto's sacrifice in his previous life weakened the seal, and now this evil has emerged. It could be a powerful demon or a dark ninja with extremely strong and malevolent jutsu.

It could be Orochimaru. Since Naruto was a potential threat to his plans in the original story, if Naruto died as a baby, Orochimaru might be able to carry out his experiments and schemes more freely without having to worry about Naruto interfering in the future.

It could be an unknown shinobi from a far - off land. A shinobi with powers that Naruto has never encountered before. This new antagonist might have some dark and powerful jutsu that Naruto simply isn't prepared for, which ultimately leads to his downfall on the bridge.

A rogue ninja from another village could be the main antagonist. Maybe a ninja from the Hidden Mist Village who has a personal grudge against Konoha and sees Naruto as the symbol of Konoha's strength. So, he devises a complex plan to take down Naruto. This ninja could have some unique and deadly jutsu that Naruto has never faced before, which ultimately leads to his downfall.

It could be an evil spirit that wants to use Naruto's new ghostly form for its own dark purposes. Maybe it tries to control Naruto and make him do evil things.

It could be Orochimaru. He's always been interested in dark and forbidden jutsus. He might see Naruto as an interesting experiment and use some dark magic on him to turn him into a berserker just to see what chaos he can cause in the ninja world.