In some fanfictions, Ash might become a champion by teaming up with his old friends in a grand tournament. They all bring their strongest Pokemon and strategies, and Ash emerges as the overall champion. It's also common to see a storyline where Ash discovers a new and powerful Pokemon that helps him achieve the champion status.
Well, perhaps Ash starts to idolize Pikachu after seeing its unique powers in a battle. He might try to imitate Pikachu's moves and even start collecting Pikachu - themed items.
In the 'pokemon ash becomes a charmander fanfiction', it could be that Ash witnesses a Charmander perform an incredibly heroic act. For example, saving a group of Pokemon from a dangerous situation. This act of selflessness and courage makes Ash look at Charmander in a whole new light. He starts to study Charmander's behavior, training techniques specific to Charmander, and even starts collecting Charmander - related items. He becomes so obsessed that he starts to model his whole Pokemon - training approach around what he has learned from Charmander.
Hau might be his toughest rival. Hau has a friendly rivalry with Ash throughout the Alola journey. He's also quite skilled and has some unique Pokémon that could give Ash a hard time in the championship battle.
One idea could be Ash facing off against old rivals who are jealous of his new Kalos champion status. They challenge him in unique battles that test his skills in new ways.
It could be a trainer who has a dislike for Charmander. This trainer might see Ash's new - found love for Charmander as a threat and try to undermine Ash's relationship with his Charmander. For example, this antagonist could try to steal Charmander or set up difficult battles where Charmander is at a disadvantage.
Ash would likely be shocked at first. He's so used to seeing the good in people, especially fellow trainers. But then he'd probably get angry and determined to set things right. He has a strong sense of justice, so he wouldn't let the betrayal go unpunished.
A fellow trainer in Alola could betray Ash. This trainer might be envious of Ash's success as a champion. He could secretly work with Team Skull or some other antagonists to undermine Ash. For instance, he might give false information to Ash about a battle location, leading Ash into a trap.