Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
One interesting crossover could be Ash and his Pokemon traveling to the Naruto universe and having to deal with the ninja world's challenges. Maybe Pikachu uses its electric attacks against ninja techniques.
In some fanfictions, they might explore how Ash's bond with his Pokémon is similar to Naruto's bond with his friends in the Hidden Leaf Village. Ash's journey to become a Pokémon master and Naruto's journey to become Hokage both involve growth, friendship, and facing challenges. They could learn from each other's experiences. For instance, Naruto could teach Ash some of his ninja stealth techniques which could be useful when catching Pokémon in the wild.
When comparing their powers more closely, Ash's Pokémon can cover a wide range of elements and abilities. For example, a Charizard can fly and breathe fire, which gives Ash an advantage in aerial combat and against foes weak to fire. Naruto's powers are centered around his own body and chakra control. His Rasengan is a powerful close - range attack that can cause a lot of damage. However, if Ash's Pokémon can keep their distance, they could potentially outmaneuver Naruto. But Naruto's speed and agility from his ninja training could also give him an edge in dodging Pokémon attacks.
Ash could be a Pokémon trainer with a different starting point in this fanfiction. Born in 1987, by 1997 he might have already made a name for himself in local Pokémon battles. He could have a team of Pokémon that he rescued from difficult situations, like a Charmander that was abandoned by its trainer. His story could revolve around giving these Pokémon a second chance and proving that Pokémon don't need to be 'perfect' to be strong.
The impact of Ash's loss in the fanfiction could extend beyond just himself. His Pokémon might also feel demoralized. However, this could also be an opportunity for them to grow stronger together. Ash could start training more intensively, seek new training methods, or even try to catch new Pokémon to strengthen his team.
One popular type of Pokemon Ash and Pikachu fanfiction might be adventure - based. For example, Ash and Pikachu could embark on a journey to a new and mysterious region filled with undiscovered Pokemon species. They might face challenges like difficult terrains and powerful, previously - unseen Pokemon. In this fanfiction, Pikachu's electric powers could be enhanced in some unique way, and Ash would have to strategize differently to overcome the obstacles.
Maybe Ash gets lost in the Naruto world and Sakura finds him. At first, she's suspicious, but as she learns about his journey with Pokemon, she decides to help him find a way back. Along the way, they exchange knowledge. Ash teaches her about different Pokemon types, and Sakura shows him some new ninja techniques.
Well, in certain fanfictions, Ash's inexperience in handling different Pokémon species can make him seem like a bad trainer. He may not be aware of the specific training methods required for some Pokémon. For instance, he might treat a very delicate and rare Pokémon the same way he treats his more common ones, which could lead to the Pokémon not reaching its full potential.
One popular aspect of Alola fanfiction with Ash could be his growth as a trainer. In the Alola region, Ash had unique experiences. Fanfiction might explore how he further developed his skills, formed new bonds with Pokemon, and interacted with the local culture. For example, his relationship with Pikachu might be delved into deeper, showing how they overcame new challenges together in the Alola setting.