There could be a group of rivals rather than just one. A team of young Johto trainers who idolize the previous Johto League champions. They see Ash as a threat to their dreams of following in their idols' footsteps. They might gang up on Ash during battles or try to sabotage his journey in more subtle ways, like spreading false rumors about him in the Pokémon Centers. This would force Ash to not only focus on his Pokémon training but also on his reputation management.
He might encounter rare Pokémon that are unique to the Johto region, like Chikorita or Cyndaquil as his starter. He could also face off against the infamous Team Rocket, who always seem to be up to no good in the Pokémon world.
Even some of Naruto's former allies could turn into rivals in fanfiction. Maybe due to a misunderstanding or a new power - hungry ambition, they start to compete with Naruto for the title of master of the elements. This would add an interesting twist to the story as Naruto has to deal with not only external threats but also internal strife among his once - close comrades.
Some potential rivals could be the local Unova trainers who are known for their unique battling styles. For example, a trainer who specializes in using Bug - type Pokemon could be a tough rival for them.
One potential rival could be a male trainer who has a more traditional view of Pokémon training and looks down on the fem Ash's different approach. He might constantly try to one - up her in battles.
A self - proclaimed 'collector' of rare pokemon could be a rival. This person has a huge collection of non - shiny but very rare pokemon and sees Ash's shiny pokemon as the ultimate addition to their collection. They would stop at nothing to get their hands on at least one of Ash's shiny pokemon. And of course, there could be other trainers who simply can't stand the fact that Ash has such an amazing ability and they don't.
His first Sinnoh Pokémon could be a Chimchar. Imagine Ash finding Chimchar being bullied by some other Pokémon. Ash steps in and Chimchar decides to join him. Chimchar's energetic and mischievous nature would add a lot of fun to Ash's Sinnoh adventures as it grows and evolves into an Infernape.
They might encounter rare Pok茅mon and face tough battles. Maybe they also make new friends along the way.
Well, consider Ash starting in the Hoenn region in a fanfiction. The Pokémon in Hoenn are known for their diverse abilities and forms. His Pokémon from other regions might feel a bit out of place at first. But as they explore Hoenn together, they would learn to adapt. His relationship with his Pokémon could become more collaborative as they all face new challenges. For instance, his Pikachu might have to deal with stronger Electric - type Pokémon in Hoenn, which would make it rely more on Ash for training and support. His other Pokémon would also have to learn new techniques to compete, and this shared struggle would deepen their bond with Ash.
Sasuke could be a rival in this fanfiction. Sasuke has always been strong in fire - based jutsu himself. He might see Naruto's claim as the master of fire as a challenge and want to prove his own superiority. Another could be some of the members from other ninja villages who have also mastered powerful fire techniques. For example, a ninja from the Hidden Rock Village who has a unique and powerful fire - based kekkei genkai.
It really depends on the specific plot and character development in the fanfiction. Sometimes, it could be after undergoing intense training or having certain key experiences.