It could be a powerful dragon hunter. This person would see Naruto's dragon as a prize to be captured or killed. They might have a personal vendetta against dragon riders in general.
A powerful dark wizard could be the main antagonist. He might be jealous of the dragon rider's powers and the bond with the dragon, and wants to break that bond and steal the rider's powers for himself.
It could be Orochimaru. He is always seeking power and the strange and powerful abilities of the Ghost Rider might be something he desires. He could try to capture or study the Ghost Rider to gain an edge in his own plans for power and immortality.
It could be a dark wizard who wants to control the dragons for his own evil purposes. He might see Harry as a threat because Harry is forming a bond with the dragons and teaching them to resist dark magic.
Another possibility is a corrupted dragon spirit that has been awakened due to the mixing of the two worlds. This spirit could be spreading chaos and trying to destroy both the ninja world and the dragon world. Naruto would have to figure out a way to stop it, as it has powers that are beyond what he has faced before.
There could be a powerful sorcerer who is jealous of the unique bond between Naruto and the dragon maids. This sorcerer wants to break that bond and take control of the dragon maids' power for himself. He uses dark magic to create illusions and obstacles for Naruto and the dragon maids, trying to turn them against each other so he can achieve his evil goals.
There could be a group of dark ninjas who see Naruto's connection with the dragon as a threat. They believe that Naruto and the dragon together could become too powerful and disrupt their evil plans. So, they set out to destroy the dragon and capture Naruto to prevent them from achieving their goals.
Perhaps a powerful sorcerer who wants to capture the black horse for his own dark purposes.
It could be Orochimaru. He might have heard about the unique magic in Dragon Academy and wants to use it for his own evil purposes. He could try to corrupt the magic there and use it to enhance his jutsu or gain immortality in a new way.
It could be a powerful ninja who wants to control the one - eyed dragon for its power. This ninja might be jealous of Naruto's connection with the dragon and seek to break that bond and take the dragon for himself.
It could be a rogue ninja who has also mastered the Fire Dragon Roar but uses it for evil. This ninja might have a grudge against the Hidden Leaf Village and wants to destroy it using his powerful fire - based attacks. Maybe he was once rejected from the village and now seeks revenge.