A possible main villain could be an evil ninja who has somehow managed to harness dark magic similar to that which the Monkey King once faced. He could be using this magic to try and take over both the ninja world and the world of the Monkey King.
Maybe a corrupted spirit that has the power to manipulate time. It wants to rewrite history in both universes so that it can rule supreme. It would be a very difficult enemy for Naruto and the Monkey King to face as they would have to deal with time - related challenges.
It could be Orochimaru from Naruto who somehow finds his way into the Lion King world. He would be interested in the unique magic and power of the Pridelands. He could try to control the lions or use them for his evil experiments, like he did with his human subjects in Naruto.
It could be a powerful demon that has the ability to absorb the powers of others. This demon has been observing both Gohan and Naruto from the shadows and wants to take their powers to become invincible.
A new villain could be created specifically for this fanfic. Maybe an interdimensional being that senses the strange energy fluctuations caused by Gohan's training of Naruto. This being, fearing the new power that Naruto might gain, decides to invade Naruto's world to stop the training and take over the world for itself.
It could be Frieza. Frieza is a powerful and evil character from Goku's universe. He might see the combined power of Naruto and Goku as a threat and try to eliminate them. He could also be interested in the unique powers that Naruto has, like the Nine - Tailed Fox's chakra, which he might want to use for his own evil purposes.
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.
Maybe a powerful demon that has been lurking in the shadows of both universes. This demon is attracted by the unique magic in both Coraline's and Norman's worlds. It starts to possess the people and creatures in both places, and Coraline and Norman must find a way to exorcise it. The demon could be very cunning, disguising itself as friendly beings at first.
The Monkey King has many adventures. For instance, he rebels against the Jade Emperor in heaven. He fights against various powerful deities there. Then, during the pilgrimage to the West, he uses his magical powers like the 72 transformations to overcome difficulties. He also battles with powerful demons such as the Bull Demon King. His adventures are full of challenges and his cleverness and strength are constantly tested.
In the Monkey King story, the central figure is the Monkey King. He is a powerful and mischievous character. Then there's Tang Sanzang, a Buddhist monk. Tang Sanzang is the one that the Monkey King protects during their journey to the West. Zhu Bajie is another character. He is often depicted as lazy and gluttonous but also has his own charm. Sha Wujing is the most loyal and hard - working among them. All these characters together create a vivid and interesting story.
Obviously, the main characters are the king and the monkey.
Maybe an ancient evil spirit that has been awakened due to some mistake. It targets Ageha because she has a pure and powerful chakra. Naruto, being the hero he is, tries to fight the spirit, but he quickly realizes that he needs Ageha's help as well as the help of all his friends to defeat this powerful foe.