It could be an ancient demon that was sealed away long ago in the Land of Demons but has now broken free. This demon could be extremely powerful and have a vendetta against the ninja world.
Maybe a rogue ninja who has made a pact with the demons in the Land of Demons. They seek to use the demons' power to take over the ninja villages, and Naruto has to stop them. This antagonist would have both ninja skills and demonic powers at their disposal.
A powerful shaman from the Land of Demons who believes that the presence of ninjas like Naruto is a threat to their land. They use dark magic and summon powerful demons to drive out the intruders, and Naruto has to convince them otherwise or defeat them to ensure peace between the two worlds.
A group of bandits who have made the Land of Snow their base. They constantly raid the villages and Naruto has to stop them. These bandits could have some sort of special ability related to the cold environment, like being able to blend in with the snow or use ice weapons more effectively than normal people.
The main antagonist could be a demon who is threatened by Naruto's singing ability. This demon might be using people's inner demons to gain power, and Naruto's singing that calms those demons is ruining its plans.
He might meet a powerful demon king who initially sees Naruto as an intruder but later realizes his good intentions. There could also be a young demon who is curious about Naruto and helps him navigate the land.
It could be the Nine - Tailed Fox. Since it already has a connection to Naruto, it might take him under its wing in a more fatherly or family - like way. Another possibility is some of the lesser - known demons from the Naruto lore that haven't been explored much.
It could be the other demons who oppose the Queen of Demons' newfound alliance with Naruto. They might see Naruto as a threat to their way of life or as a tool that the Queen of Demons is misusing. So, they would try to break their bond and destroy Naruto.
A rogue group of transformed dinosaurs that have lost their sense of morality due to the transformation could be the main antagonists. They might use their new powers to take over the best feeding grounds and force the other dinosaurs out.
It could be a powerful dark wizard who has found a way to control the forces in the Land of the Dead. Maybe he was once a student at Hogwarts who delved too deep into the Dark Arts and now wants to use the power of the dead to rule the wizarding world.
Well, one possible plot could be that Naruto goes to the Land of Demons in search of a powerful demon that has been causing trouble in the ninja world. He might face various challenges there, like tricky terrains and demon minions. Maybe he has to team up with some local demons who are also against the evil one.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
It could be a powerful demigod who has been corrupted by the God of Demons' power. This demigod might have his own agenda and wants to use the chaos caused by the God of Demons to take over the demigod world.