There's also the possibility that the main antagonist is not a single person but a group. A group of rogue ninjas who oppose the current peace in the ninja world and Naruto has to stop them from causing chaos. They could be led by someone who has a grudge against Naruto or the way he brought peace in the past.
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.
Maybe an old enemy that Naruto thought was defeated has come back stronger. For example, Orochimaru could have found a way to resurrect himself in a more powerful form and now wants revenge on Naruto and the Leaf Village.
There could be a group of anti - ninja revolutionaries as the main antagonists. They see Naruto's return with the swords as a threat to their plan of overthrowing the existing ninja order. They try to take the swords from Naruto to use them as symbols of their new regime, believing that the power of the swords will help them convince other ninjas to join their cause.
A new dark entity could be the main antagonist. This entity might have been awakened by Naruto's return from the dead. It could be something that was sealed away long ago and Naruto's return breaks the seal. The entity would then target Naruto and his loved ones to prevent him from getting in its way of ruling the ninja world.
It could be an unknown ninja who has been researching dark and forbidden arts. This ninja accidentally unleashes the Black Death - like plague while trying to gain ultimate power.
Perhaps an old enemy that Naruto thought was defeated has returned, stronger than ever. This enemy could hold a grudge against Naruto and the elemental nations for past defeats and wants revenge.
Perhaps it's an old enemy who has somehow managed to come back stronger. Maybe someone from Orochimaru's old experiments that went wrong, but instead of dying, they gained some dark and powerful abilities. They see Naruto's return as a threat to their plans of world domination, so they target Konoha first. They might be able to control dark chakra in a way that no one has seen before, and use it to create chaos and destruction. They could also have some minions who are equally as powerful and loyal to them, making them a formidable foe for Naruto and Konoha.
It could be a rogue ninja from a neighboring village who has a vendetta against Konoha. This ninja might see Naruto's return as an opportunity to take on the strongest ninja in Konoha and prove his own strength.
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.
A new character could be the main antagonist. This character could be from a far - off land that Naruto has never encountered before. This antagonist has heard of Naruto's power and wants to prove himself by defeating Naruto and taking over the Hidden Leaf. He might have unique powers and a large army at his disposal. He could also be manipulating some of the villagers from the inside to create chaos before making his big move.
The main antagonist could be an ancient evil that has been awakened. Harumi returns to find Ninjago in chaos because of this new threat. She may initially be blamed for the chaos, but as the story unfolds, it becomes clear that the real villain is this ancient evil which the ninjas and Harumi must then team up to defeat.
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.