It could be an evil fairy who was exiled from the fairy - tale world long ago. When Cory enters the Shrek world, her magic senses pick up on his 'foreign' energy. She sees this as an opportunity to get revenge on the fairy - tale world. She tricks Cory into helping her, making him think she's good. But once he realizes her true intentions, he has to team up with Shrek and his friends to stop her from casting a dark spell that would plunge both worlds into eternal darkness.
One possible plot could be that Cory somehow gets transported to the world of Shrek. Maybe he's on a mission from the White House to find a special magical item that's rumored to be in Shrek's swamp. Along the way, he teams up with Shrek and Donkey. They face challenges like trying to outwit Lord Farquaad, who also wants the item for his own evil plans. Cory uses his street - smart skills from living in the city to help them navigate through tricky situations.
Perhaps it's a new character, like an evil wizard who has heard about the power of Shrek's family. He believes that by getting rid of the ogre babies, he can gain control over the swamp and all of Far Far Away. So he devises various evil plans to harm them.
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.
Perhaps an ancient evil spirit that has been awakened by the mixing of the two worlds - Ariel's underwater world and Mowgli's jungle world. This spirit wants to plunge both worlds into darkness. It starts by causing natural disasters in both areas, like huge storms in the ocean and forest fires in the jungle. Ariel and Mowgli must figure out a way to stop this spirit before it's too late, by finding ancient artifacts that can seal the spirit away again.
There could be a group of rogue Huntsmen who believe that the current system of protecting the world is flawed. They form a radical organization and start causing chaos. Their leader, who is a very powerful and charismatic character, could be the main villain. They would have their own set of ideals and be willing to do whatever it takes to achieve their goals, even if it means going against the established order.
A jealous ex - employee of Game Shakers could be a great villain. They were fired for some reason, like being caught stealing ideas, and now they want revenge. They could try to sabotage the company's new projects and spread false rumors about the Game Shakers.
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.
It could be a new character, a powerful dark wizard who wants to use the combined power of Naruto's chakra and Pikachu's electricity for his own evil purposes. He creates all sorts of dark creatures and sends them after Naruto and Pikachu to try and capture them. He's very cunning and has a lot of minions to do his bidding.