One possible plot could be that Ash wakes up one day to find he has Pikachu ears and a tail. He initially panics but then realizes he can use these new features to better understand Pikachu. For example, he discovers he can sense electricity in a similar way to Pikachu, which helps them in battles. His friends are both shocked and amused at first, but they soon see the advantages.
In this fanfiction, Ash's personality might change in a way that he becomes more in tune with nature and other Pokémon. The Pikachu ears and tail could give him a different perspective on the world of Pokémon. He may start to communicate more like Pikachu, using simple sounds and gestures. This could make him seem a bit more wild or untamed at first, but as he learns to control his new form, he would blend his old personality with the new traits, becoming a more complex and interesting character.
The change in its appearance can make it scary. For example, if its usually cute yellow fur turns dark or if its bright red cheeks become a sinister red glow. Also, if it starts to behave in an uncharacteristic way, like making menacing noises instead of its usual cute sounds.
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.
One interesting plot could be two Pikachus from different regions meeting and learning about each other's unique electric attacks. For example, one Pikachu might know a special move that the other has never seen before. They could then teach each other and become stronger together.
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.
Lucario is also a popular choice to be paired with Pikachu in crossovers. Lucario has its own unique abilities related to sensing and combat. When combined with Pikachu's electric attacks, they can form a formidable team. The contrast between Lucario's more serious and stoic nature and Pikachu's cute and energetic personality also adds depth to the story.
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.
Brock could be a great addition. He could provide some comic relief and also be the voice of reason. For example, when Ash and Pikachu get into some crazy situations, Brock could be the one to offer practical solutions.