Perhaps the twist is that Ash's transformation into Charizard is a sign of a greater prophecy. In the fanfic, there could be an ancient legend that says when a human turns into a Charizard, it means a great danger is coming. So Ash has to figure out what this danger is while dealing with his new form. And as he tries to find answers, he discovers that his bond with Pikachu and other Pokémon is the key to both solving the mystery and getting back to his normal self.
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.
A potential plot twist might be that Ash's transformation into Charizard is only temporary, but every time he transforms back and forth, he gains some new ability or power. For example, he could start to understand Pokémon language better. Also, it could be that the Charizard form has a mind of its own, and Ash has to fight for control, which would add a lot of internal conflict to the story.
Chapter 266, "He Really Deserves to Die!", is the clearest early answer because it turns Chixia Mountain's situation into a concrete plot event. Early on, Upon hearing this, the deputy director's expression also turned....
Chapter 501, "This Night, Destined to be Turbulent", is the clearest early answer because it turns Void's situation into a concrete plot event.
Chapter 373, "Mr. Jin is the Best, How Could He Possibly Lose?!", is the clearest early answer because it turns Possibly Lose's situation into a concrete plot event. Early on, As night fell, the lobby of the Golden Sand lit up with layers of....
Chapter 373, "Mr. Jin is the Best, How Could He Possibly Lose?!", is the clearest early answer because it turns Jin's situation into a concrete plot event. It opens with As night fell, the lobby of the Golden Sand lit up with layers of...
Chapter 763, "It must be Jiang Li who tied her up!", is the clearest early answer because it turns Miss Su's situation into a concrete plot event. The chapter turns that into something concrete by moving from But within a short time, she sensed that something was off.
Chapter 284, "You'll Never Get Rid of Me!", is the clearest early answer because it turns Lin Manru's situation into a concrete plot event.
Chapter 784, "Slander Brother Yang Without Any Proof?!", is the clearest early answer because it turns Brother Yang's situation into a concrete plot event.
Chapter 613, "When a Whale Falls, All Life Thrives", is the clearest early answer because it turns Shen Ruobing's situation into a concrete plot event. Early on, Hearing He Wen's words, Shen Lan was so excited she nearly sprayed....
Chapter 374, "From Now On, Jiang Li Calls the Shots in Golden Sand!", is the clearest early answer because it turns Jiang Li Calls's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Li's bold behavior infuriated Jin Siyue, but it also succeeded...
Chapter 285, "Ksitigarbha Bodhisattva", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. The chapter turns that into something concrete by moving from He could distinctly feel his back rubbing intimately against the...