Another option could be a new character created specifically for this fanfiction. This character could have the ability to manipulate both the ninja and the Ghost Rider elements, and wants to use that power to rule over both worlds. So, they would be the main adversary that Naruto and others have to face.
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.
Perhaps an evil spirit that has been luring the ghost child into doing bad things. This evil spirit could be jealous of the connection the ghost child has with Naruto or the living world. It wants to corrupt the ghost child and use it to spread chaos in the Naruto world. It could be a spirit that was once a dark ninja who was never able to fully pass on and now wants to cause destruction through the ghost child.
Perhaps in this fanfiction, there could be a scenario where Naruto has to team up with some of his friends to take on Ghost Rider. They would have to figure out Ghost Rider's weaknesses, which could be a mix of things from both the Naruto universe and what we know about Ghost Rider. And along the way, they might learn some new things about their own powers too.
It could be Orochimaru from the Naruto universe. Imagine he finds a way to cross over to the Kamen Rider world. He would be attracted by the unique powers there and want to use them for his own evil experiments. His ability to manipulate and deceive would make him a formidable villain against both Naruto and the Kamen Riders.
The main characters in 'naruto legend of the ghost rider fanfiction' are hard to determine without reading it. However, based on the title, Naruto is a prime candidate. It's possible that the story might introduce a new character who is the Ghost Rider in the Naruto universe. This character could interact with Naruto and his friends. It could also be that some of the more morally complex Naruto characters, such as Orochimaru, might play significant roles, either as allies or antagonists, especially considering the dark nature of the Ghost Rider theme.
Well, one way could be Naruto having the Ghost Rider's chain weapon. He could use it in his combat style, like when he's facing tough enemies. Maybe the chain could be enhanced with his chakra, giving it new abilities.
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 from the human world who has found a way to cross over into Equestria. He wants to harness the magic of the Elements of Harmony to increase his own power. He sees the Ghost Rider as a threat to his plans and tries to eliminate him while also causing chaos in Equestria.
It could be Orochimaru. He's always interested in new forms of power and experimentation. He might see Wolverine as an interesting specimen to study and try to extract his healing factor or adamantium for his own purposes. This would put him in direct conflict with both Wolverine and Naruto, who would naturally oppose Orochimaru's evil plans.
Danzo could be a great main villain in a Naruto Frankenstein fanfiction. He's always been power - hungry and secretive. He might use the concept of creating a Frankenstein - like being to gain more power for Konoha in his own twisted way, but really it's all about his own control. He could be using parts of dead ninja's abilities or something like that, and Naruto would have to stop him.