One possible plot could be that Ash is forced to give up Charizard in a high - stakes Pokémon battle. Maybe a powerful trainer challenges Ash and demands Charizard as the prize if he loses. Ash, being a proud trainer, doesn't expect to lose but due to some unforeseen circumstances, like a new and unknown Pokémon move used by the opponent, he loses the battle. Charizard, being sent to the new trainer, initially resists but is somehow forced to obey. This makes it seem like Charizard has betrayed Ash as it has no choice but to follow the new trainer's commands.
One possible plot could be that Ash and Brock are lured into a false sense of security by a powerful new trainer. This trainer seems friendly at first but then secretly uses their Pokémon to undermine Ash and Brock during important battles. Ash and Brock might start to suspect something when their Pokémon keep getting mysteriously weakened. Eventually, they realize they've been betrayed when the trainer reveals their true intentions of trying to take over their region's Pokémon league.
One possible plot could be that Cynthia, perhaps in a quest for power or influenced by some dark force, betrays Ash. Maybe she steals his Pokémon to use in her own battles or to offer to a powerful villain in exchange for something she desires, like rare Pokémon or access to a powerful training ground.
Ash would be heartbroken. He has a deep bond with Charizard, and seeing it turn against him would be like losing a part of himself. He would probably first try to reason with Charizard, thinking that there must be some mistake. When that doesn't work, he might become a bit angry, but mostly he'd be sad and determined to find out what's wrong and fix it.
The plot might involve Ash starting his journey all over again, but this time with a Charizard as his very first Pokémon. Since Charizard is a powerful and somewhat stubborn Pokémon, Ash would have to learn how to handle it right from the start. He could face challenges like other trainers trying to steal Charizard because of its rarity. Ash would have to prove his worth as a trainer and his connection with Charizard to keep it by his side.
The plot could be that Ash is orphaned at a young age and Charizard finds him. Charizard then decides to raise Ash in the wild. Ash grows up learning survival skills from Charizard and how to communicate with other Pokémon directly.
The plot could be that Ash enters a Kalos - only Pokémon competition with Charizard. They face strong opponents with unique Kalos Pokémon. Charizard might learn new moves by observing the local Pokémon. For example, it could see a Delphox use a powerful fire - type move and try to adapt it to its own style.
The plot might revolve around Ash leaving Charizard because he believes Charizard has a greater purpose elsewhere. For example, there could be a group of wild Charizard in need of a leader, and Ash realizes that his Charizard is the one. So he leaves it to fulfill that role. In the fanfiction, we could see Charizard initially being reluctant but then gradually accepting its new destiny and leading the group in a new and better way.
Well, perhaps Ash has returned to Kanto for a Pokemon tournament. As he's exploring the old stomping grounds, he stumbles upon Charizard who has been living in the Charicific Valley. At first, Charizard is a bit hesitant, but then it recognizes Ash. They then go on a journey together through Kanto, revisiting old friends like Brock and Misty, and along the way, they have to deal with some rogue Team Rocket members who are up to their old tricks. This reunion rekindles their unbreakable bond.
Well, it could be about Charizard being Ash's really loyal and powerful companion in some epic Pokémon adventures. Maybe they face off against some super-strong villains and Charizard uses its fire - type moves to save the day for Ash.
One possible plot could be that the female Charizard has a special ability that no other Charizard has. Ash discovers this during a tough battle, and they form an even deeper bond as they train together to master this new ability.