Who could be the main antagonist in a 'Naruto adopted by a dragon fanfiction'?

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.

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.

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.

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.

It could be a powerful dragon who has been corrupted by dark magic. This dragon might see Naruto's newfound powers as a threat and decides to target him and the Hidden Leaf Village first.

It could be an evil shinobi who has somehow harnessed shadow dragon slayer magic for his own evil purposes. He uses it to create chaos and destruction in the ninja world, and Naruto and the dragon slayer have to stop him.

It could be a rogue ninja who has also somehow gained crystal dragon slayer powers but uses them for evil. He might be jealous of Naruto's natural talent and popularity, so he tries to destroy Naruto and take over the ninja world using his newfound powers.

It could be a dark ninja who wants to control the power of the fire dragon for his own evil purposes. He might try to steal the dragon's chakra or use it to destroy villages.