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.
Well, in many fanfictions, Ash's training before going to Johto can vary greatly depending on the author's imagination. Sometimes it's intense and focused, while other times it's more laid-back and includes unexpected adventures.
It would change his journey a great deal. Firstly, having a shiny Charmander would make him stand out more. Other trainers would be more likely to challenge him, either to prove themselves against someone with a rare Pokémon or to try and get his Charmander. Also, trainers with more experience might offer to help Ash train his shiny Charmander, which could lead to new training techniques. In Pokémon contests, his shiny Charmander would be a show - stopper, giving him an edge in those aspects of his journey as well.
It could be a new character created specifically for the fanfiction. A trainer with some super - rare and powerful Pokémon that Ash has never faced before. This trainer might have unique battle tactics that really test Ash's skills.
Well, it could involve Ash using his Pikachu in some really creative strategies to defeat tough opponents. Maybe he finally overcomes his rivals like Gary in a more intense rematch in the Johto League.
Ralts could make Ash more empathetic. Since Ralts is a psychic - type Pokémon that can sense emotions, Ash might become better at understanding the feelings of other Pokémon and people. This could help him in his journey to be a better Pokémon trainer.
The Ultra Beast could completely change the direction of Ash's journey. It might introduce him to new regions or dimensions he has never explored before. For example, if the Ultra Beast has the ability to open portals, Ash could end up in places with new Pokémon species to discover and new challenges to overcome.
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.