There could be a group of hellish bureaucrats who are the antagonists. They are the ones who sent Chloe to hell in the first place and are trying to keep her there to maintain the order of hell as they see it. They might not want Chloe to expose the unjust system that exists in hell.
Perhaps Chloe is wrongly judged in some sort of afterlife court and is sent to hell. Once there, she meets other souls who help her realize that there's a conspiracy behind her being sent there. So she starts to fight against the system of hell to prove her innocence.
It could be a new villain created specifically for the fanfiction. Maybe someone who has a vendetta against Chloe and targets her because she has the Miraculous.
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.
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 Draco Malfoy. He might be jealous of Harry getting certain important mail, like letters from Dumbledore or invitations to exclusive wizarding events. So, he uses some underhanded means to make Harry's mail disappear. Draco would think it's a funny prank at first, but when Harry starts to fight back, it turns into a more serious conflict between them.
It could be a powerful dark mage who controls the tunnels and doesn't want the priestess to succeed in her mission.
An alien entity that has been dormant beneath Smallville for centuries could awaken. This entity could view Chloe and Clark as threats to its plans of taking over the town and the world. It could use its advanced powers to manipulate the townspeople and turn them against Chloe and Clark, making it a formidable foe.
It could be Voldemort or his followers. They might see Dumbledore as a major obstacle in their path to power. So they could manipulate Harry, perhaps through some mind - control magic or by spreading false rumors, to get Dumbledore fired. By removing Dumbledore, they hope to gain more control over Hogwarts and the wizarding world in general.
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 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.