It could be an escaped soul from the King of Hell's domain. This soul might have a grudge against Naruto or the ninja world in general. Maybe it was wrongly imprisoned and now wants revenge by causing chaos in the living world.
It could be the Devil or some powerful demon lord in hell. This antagonist would try to keep Naruto trapped in hell and prevent him from escaping. He might see Naruto as a threat to his rule in hell or just enjoy tormenting him.
It could be an ancient demon who has ruled hell for centuries. This demon might see Naruto's attempts to bring change and order to hell as a threat to his power. So, he tries to stop Naruto at every turn, using all the dark powers at his disposal like summoning hordes of hellish creatures or casting powerful curses.
One possible main antagonist could be the ruler of hell. This powerful being would see Naruto as an intruder and a threat to the order of hell. Another could be a demon who has a vendetta against Naruto for some reason, perhaps because Naruto's positive energy is anathema to the demon's evil nature.
It could be someone who misuses the power of the Hell Girl. Maybe a rogue ninja who discovers the way to summon the Hell Girl and uses her to get rid of his enemies in the ninja world. He doesn't care about the consequences or the morality of it, and Naruto has to stop him from causing more chaos.
It could be an evil ninja from the Naruto world who has somehow gained the power to traverse into the King of Heroes realm. This ninja, with his newfound powers and his thirst for domination, starts causing chaos in both worlds. He wants to use the power of the heroic spirits for his own evil purposes and sees Naruto and the King of Heroes as his main obstacles.
Perhaps an organization similar to Akatsuki. They might see the king of beasts as a powerful tool to achieve their goals, whether it's collecting all the tailed beasts or just ruling the world with an iron fist. They would send their strongest members to try and capture the king of beasts, and Naruto would have to stop them. In this fanfiction, this antagonist organization could have new and more powerful abilities compared to the original Akatsuki in the Naruto series.
A group of outcast animal clans could be the main antagonists. They were exiled from the main animal kingdom for their violent and power - hungry ways. Now they have united and plan to take over the entire animal world and overthrow the current Animal King. Naruto gets in their way as he is trying to protect the balance between the human and animal worlds, and they see him as a major threat.
Madara Uchiha could also be a great antagonist. Imagine if he came back and thought that the power of the Soul King could be used to achieve his ultimate goal of creating a world under his control. He'd be a formidable foe for Naruto, especially since he has so much experience and power already.
Maybe a disgruntled former member of the Glee club who feels wronged. This person has a vendetta and uses their knowledge of the club's weaknesses. They lurk on the 'highway to hell' and try to sabotage the club's journey, for example by stealing their sheet music or spreading rumors among the members to cause internal strife.
The main antagonist could be the Devil himself. He might see Harry as an intruder in his domain and try to keep him there forever. He could set up all kinds of traps and challenges for Harry to overcome.