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.
Maybe an evil Pokemon trainer who has been corrupted by the power of the Sharingan that he somehow obtained. He wants to use Pikachu's power to enhance the Sharingan's abilities even further. He hunts down Naruto and Pikachu across different regions, setting traps and using his powerful Pokemon to try and stop them at every turn.
Naruto would probably be really excited. He'd treat Pikachu like a new friend and teammate. He might try to learn all about Pikachu's powers and how it can help in battles. For example, he'd be eager to see Pikachu use Thunderbolt in a fight against an enemy ninja.
In 'naruto and pikachu fanfic', often we see a cultural exchange of sorts. Naruto's values as a ninja, like loyalty, hard work, and protecting his friends, can be contrasted and combined with Pikachu's more innocent and friendly nature. This can lead to some heart - warming moments in the fanfic. For example, Pikachu might help Naruto understand the importance of having fun in the middle of all the serious ninja business. And Naruto could teach Pikachu about the importance of training and discipline. Also, the power combinations are really cool. Naruto's ability to use chakra and Pikachu's electric attacks can be merged in unique ways, like Naruto using his chakra to enhance Pikachu's electric blasts for a super - powerful attack.
Obviously, the main characters are usually Pikachu. But sometimes, there could be other Pokémon that play important supporting roles. For instance, a Pichu could be a sidekick or a younger Pokémon that the Pikachus are protecting.
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 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.
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 Pokemon itself. Maybe a dark - type Pokemon with a grudge against humans and Pikachu in particular. This Pokemon could be using its powers to cause chaos in the city, and Pikachu has to figure out its motives and stop it before it's too late.
It could be a mad scientist. The scientist has been researching ways to combine organic and electrical energy. He sees Pikachu as the perfect specimen and Samus as someone who will try to stop his unethical experiments. So he sets out to capture Pikachu and eliminate Samus.
Well, Naruto's chakra-based powers and Pikachu's electric powers can combine in some fanfictions. For instance, Naruto could use his Rasengan and Pikachu could add its electric charge to it, creating a super - charged attack that is more powerful than either of their individual attacks.