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

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 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.

There could be a secret organization that believes the golden dragons are the key to a new world order. They oppose Naruto's attempts to slay the dragons because they want to harness their power for their own, often evil, purposes. They might send their most powerful assassins after Naruto to stop him.

Maybe Akainu from One Piece (in a crossover sense within the fanfiction). He could be warped into this Naruto - Dragon Slayer world and his extreme sense of justice could be twisted into a form of tyranny. He sees the chaos and magic in this new world as a threat that needs to be eliminated, and in his overzealousness, he becomes the main antagonist, causing destruction and pitting the ninjas and dragon slayers against him.

Maybe a rogue ninja who has made a pact with a dark force. This ninja could see Naruto as a threat to his plans of ruling the ninja world, especially with his new status as a dragon warrior of the kami. He would try to stop Naruto at all costs and might have some dark powers of his own.

The dragon god itself could be the main antagonist. Given its power and the threat it poses to the Naruto universe, it would be a natural choice. It could be a being that wants to destroy the ninja world or enslave it for its own purposes.

It could be a rival YouTuber who is jealous of the attention the main character is getting as David Dobrik's adopted child. This rival might try to spread false rumors or create drama to take the spotlight away.