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.
One idea could be a crossover where Naruto and Monkey King meet in a strange new dimension. They could team up to fight a powerful common enemy. Maybe an ancient demon that threatens both their worlds.
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 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.
The main villain could be Darkseid. Imagine if he somehow finds a way to enter the Naruto universe. He would be a formidable foe for both Naruto and Supergirl. His omega beams and immense strength would be a huge challenge for them to overcome.
It could be a rogue ninja from the village with the Tenshikage. Maybe he's jealous of the Tenshikage's power and wants to take it for himself. He could be causing chaos in the village, stealing important ninja scrolls and attacking innocent villagers.
A new character could be introduced as the main villain. This character could be someone from an ancient clan that was once in control of the 10 - tailed beast but lost it. Now, seeing Naruto with that power, they want it back and will stop at nothing to get it. They could have unique powers and abilities that make them a formidable foe for Naruto and his friends.