Who could be the main antagonist in 'naruto in another world fanfiction'?

A group of rogue ninja from the other dimension could also serve as the main antagonists. They might be jealous of Naruto's power from his original dimension and want to steal it for themselves. They could be led by a charismatic and devious leader who has a personal vendetta against Naruto for some reason, like he believes Naruto will disrupt their way of life in the dimension.

It could be a powerful wizard or sorcerer in the new world who sees Ichigo as a threat to his rule. Maybe Ichigo's presence disrupts the balance of power that this antagonist has carefully maintained.

A corrupted angel could be the main antagonist. This angel, once a protector of the new world, has fallen from grace. He has amassed an army of dark creatures and is using his divine powers in an evil way. The Doom Slayer has to face this angel's holy wrath and his hordes of minions.

The Green Goblin could be a great main antagonist. Imagine in this other world, he has even more advanced gliders and pumpkin bombs. He could be ruling over a part of the city with his goblin minions, and Spider Man has to stop him from spreading his reign of terror further. His maniacal laugh and unpredictable nature would make him a formidable foe in this new world.

It could be an alternate version of Vlad Masters. In this other world, he might have achieved total domination over both the human and ghost realms. Danny would then have to face this all - powerful Vlad and find his weaknesses in this new context.

It could be a new villain who has risen to power during Naruto's absence. This villain might have a vendetta against Konoha or the ninja world in general because of something that happened while Naruto was away.

It could be a rogue ninja who has made a pact with the demons in the demon world. This ninja has turned against the ninja world and wants to use the power of the demon world to rule over all. He would constantly oppose Naruto's attempts to restore peace.

It could be a powerful kunoichi who opposes Naruto's presence in her world. Maybe she sees him as a threat to the existing power balance among kunoichi.

Another option is that there is a group in his own world that doesn't like the idea of someone coming back from another world. They might see Izuku as an anomaly and try to capture or study him. This group could be a secret organization that has its own motives, and in their attempts to deal with Izuku, they end up being the main antagonists.

Another possibility is a group of outlaws or rebels in the banished world who don't trust Percy because he's from another place. They might think he's been sent to spy on them or take over. So they would constantly try to get rid of him. They could be very resourceful and know the ins and outs of the new world better than Percy, which would make them tough opponents.