The main antagonist might be a dark wizard or sorcerer who has a long - standing feud with the dragon. When Naruto is abandoned and finds the dragon, this antagonist sees Naruto as an easy target to get to the dragon. They might try to manipulate Naruto or directly attack him to get the dragon under their control. Also, it could be a group of rogue ninjas who want to use the dragon's power to take over the ninja world and view Naruto as a hindrance since he has a bond with the dragon.
It could be a relative from his family who was behind the abandonment. Maybe they want to keep Naruto away from some family secret or power, and they will do anything to stop him from finding out the truth.
It could be the elders of his clan. They are often the ones who make decisions in the clan, and if they see Naruto as a threat for whatever reason, they could be the main antagonists. They might be afraid of Naruto's growing power or his different way of thinking that could disrupt the traditional ways of the clan.
It could be Danzo. He's always been a bit of a shadowy figure in the Naruto series. Maybe he sees Naruto as a threat to his power and influence in the village and manipulates the situation so that Team 7 has to abandon him. He would be a great main antagonist as he has the resources and the motives to cause such a rift in Team 7.
Maybe the main antagonist is the entire Iwa council. They might be the ones who order Naruto's abandonment because they fear his potential influence on Iwa. They would do everything in their power to keep him from uncovering their plans or causing unrest among the villagers.
The Akatsuki could be the main antagonist. They might see Hanzo as an easy target after Naruto abandons him and try to capture him for their own purposes, like using his knowledge of the ninja world or his unique abilities.
It could be someone from the village who doesn't want Naruto to discover the truth about his parents. Maybe they fear that if Naruto finds out certain secrets, it will disrupt the power balance in the village.
Perhaps it could be another entity related to Hagoromo. If Naruto abandons Hagoromo, there might be a counterpart or a follower of Hagoromo who sees Naruto as a threat or a traitor. This person could then become the main antagonist, trying to either bring Naruto back to Hagoromo's side or simply eliminate him.
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.