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.
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.
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.
Chapter 317, "You Can't Afford to Mess with the Person Behind Her!", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with He Pingzhang gazed at the man curled up before him, unable to...
Chapter 405, "The Person Saved Casually Turns Out to Be a Diplomat?!", is the clearest early answer because it turns Hua Country's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The medical student was completely dumbfounded.
Chapter 345, "Audition: She Couldn't Have Gotten in Through Backdoor Connections, Could She?", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event.
Chapter 311, "Signed Jiang Yan", is the clearest early answer because it turns Seeing Shen Lan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Shen Lan knocked on the door before entering when Jiang Li was...
Chapter 318, "I Shouldn't Have Given Birth to You in the First Place!", is the clearest early answer because it turns Aunt Li's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Cheng glanced at the pink and white building from the bus.
Chapter 208, "Those Who Commit Many Wrongs Will Destroy Themselves", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
Chapter 266, "He Really Deserves to Die!", is the clearest early answer because it turns Director Sun Suspected's situation into a concrete plot event. It opens with Upon hearing this, the deputy director's expression also turned...
Chapter 496, "Hot Pot Tonight!", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. Early on, This woman is way too fierce, yelling at our Wenwen like that.".... By the end, Jiang Li gathered them up, found a flat stone, and began to rub them....
Chapter 583, "Making a Fool of those Big Shots!", is the clearest early answer because it turns Zhou Xu's situation into a concrete plot event. Early on, Days before, Jiang Li had predicted that today would bring a great.... By the end, You can play the game this way?!.
Chapter 553, "His Love Runs Deep", is the clearest early answer because it turns Chen Manman's situation into a concrete plot event. Early on, His gaze shifted to Luo Xuan's empty hands, and Ai Xiaochun's.... By the end, We could use this to our advantage..