Maybe in the fanfic, Ash and his friends are exploring an ancient ruin. Charizard, being curious, wanders off and accidentally triggers a booby - trap. It gets severely injured. Ash is then on a race against time. He needs to find a way to heal Charizard before it's too late. Along the way, he meets a mysterious old trainer who gives him some advice on a rare and powerful healing method. But the method requires Ash to face his own inner demons and prove his worth as a trainer.
One possible fanfic could be that Ash and his team are in a tough battle against a powerful opponent. Charizard, being overconfident, takes on a powerful attack head - on to protect Ash and the rest of the team. As a result, it gets seriously injured. Ash is devastated and has to quickly come up with a plan to both tend to Charizard's wounds and continue the battle. He uses his knowledge of Pokemon healing techniques he's learned over the years to stabilize Charizard and then makes a strategic retreat to a nearby Pokemon center.
One possible plot could be that Ash realizes Charizard has grown too powerful for the current region he's in and decides to release it so it can find a more challenging environment. Maybe it goes off to a far - off mountain range where it battles and befriends other powerful Pokémon.
The plot could involve a young trainer starting their journey with Charizard. They might face challenges like battling against powerful gym leaders. Charizard could have unique abilities that help the trainer progress, like a special fire - based move that overcomes water - type opponents easily.
One possible plot could be that Ash's Charizard is revealed to be female in a special way. Then, as they journey together, they develop a deeper bond that goes beyond the typical trainer - Pokemon relationship. Maybe there are some challenges they face where the female Charizard's unique abilities or personality traits play a crucial role in helping Ash and his friends.
The plot could be that Ash is in Kalos to participate in a special Pokémon tournament. Misty hears about it and decides to come support him. Charizard, of course, is Ash's main battling Pokémon. Along the way, they meet a new type of Pokémon that is causing trouble in the region. They have to figure out a way to stop it, with Charizard using its powerful attacks. And in the end, Ash wins the tournament with the help of Misty and Charizard.
It could be about Ash and his Charizard training to master the blue flames. Maybe they encounter a powerful enemy that can only be defeated with this new, more powerful form of Charizard's flames.
One plot twist could be that Ash - as - Charizard loses his memory of being human. His friends have to find a way to remind him who he is. Another could be that the transformation is not a random accident but part of an evil plan by a villain who wants to use Ash's new form for their own purposes.
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, it could mean that Ash has an extremely powerful ally in the Sinnoh region. Maybe he uses Mega Charizard to battle against powerful gym leaders or face off against the evil teams more effectively. It might also attract the attention of other trainers who are either in awe or jealous of his powerful Pokémon.