There's also a possibility that the king is a time - traveling character. He could be from the future of the Naruto world, where things have gone wrong. He discovers a way to travel to the X - Men universe and starts consuming elements from both worlds in an attempt to rewrite history and become a powerful and benevolent king who can correct the mistakes of his own world.
The plot might involve the king being from a dimension where he discovers these two fictional worlds. He starts eating things that are symbolic of the X - Men and Naruto. It could be like he eats a scroll that contains the essence of Naruto's jutsus and then a device that holds the genetic code of the X - Men mutants. This gives him abilities that are a mix of both universes, and he has to deal with the consequences of having such powerful and conflicting powers.
It could be an original character created specifically for the fanfiction. Maybe a powerful wizard king from a far - off land who has heard of Harry's deeds and is intrigued by his power and courage.
One possible plot could be Naruto being transported to the Pride Lands. He could use his ninja skills to help Simba and the other lions protect their territory from a new threat, like a group of powerful and evil hyenas that have some kind of dark magic. Naruto's ability to make clones would be really useful in battle, and his determination could inspire Simba and the pride.
Obviously Naruto and Wolverine could be main characters. Wolverine's tough - guy attitude and his healing factor would contrast well with Naruto's upbeat and determined personality. Jean Grey could also be a main character. Her powerful telepathic and telekinetic abilities would be interesting to see in the Naruto world, especially when interacting with ninjas who rely on physical and chakra - based skills.
It could be a group of rogue ninjas who want to capture the dragons for their power. They might be led by a powerful and greedy ninja who sees the dragons as a means to gain ultimate power over the ninja world.
It could be an evil Digimon that has the power to control other Digimon and wants to use them to invade the Naruto world. This Digimon might be jealous of the Digimon King's power and Naruto's connection to it, so it aims to destroy both of them. It could have abilities like mind - control and dark energy attacks.
It could be an escaped soul from the King of Hell's domain. This soul might have a grudge against Naruto or the ninja world in general. Maybe it was wrongly imprisoned and now wants revenge by causing chaos in the living world.
It could be an evil ninja from the Naruto world who has somehow gained the power to traverse into the King of Heroes realm. This ninja, with his newfound powers and his thirst for domination, starts causing chaos in both worlds. He wants to use the power of the heroic spirits for his own evil purposes and sees Naruto and the King of Heroes as his main obstacles.
Perhaps an organization similar to Akatsuki. They might see the king of beasts as a powerful tool to achieve their goals, whether it's collecting all the tailed beasts or just ruling the world with an iron fist. They would send their strongest members to try and capture the king of beasts, and Naruto would have to stop them. In this fanfiction, this antagonist organization could have new and more powerful abilities compared to the original Akatsuki in the Naruto series.
A group of outcast animal clans could be the main antagonists. They were exiled from the main animal kingdom for their violent and power - hungry ways. Now they have united and plan to take over the entire animal world and overthrow the current Animal King. Naruto gets in their way as he is trying to protect the balance between the human and animal worlds, and they see him as a major threat.