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.
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 572, "Only Second in the Country", is the clearest early answer because it turns Hua University's situation into a concrete plot event. The chapter gives readers a real answer by opening on Little brother, this is a competition, and I won't go easy on you...
Chapter 331, "The Casual Video Went Viral?!", is the clearest early answer because it turns Went Viral's situation into a concrete plot event. The chapter gives readers a real answer by opening on At that moment in the He family.
Chapter 240, "He's the Last One?!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from Wuuuuh He Bao truly deserves it, Mom always knew you could do it!"...
Chapter 649, "Where Did the Wild Wolf Come From?!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Several people successively lay down in the tent, and before long,...
Chapter 614, "Storm Crisis", is the clearest early answer because it turns Ji Yanbai's situation into a concrete plot event. It opens with Today, the wind on the island was stronger, and for safety reasons,... and closes on He slightly raised his head and met Pei Ye's dead eyes, his scalp....
Chapter 803, "Younger Brother Misses Mom", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Brother, do you think hiding here is a bad idea?" "Don't worry,...
Chapter 483, "Why Does a Pig Eat M9?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
Chapter 645, "Without Luck, You Are Nothing!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Shang Shaojing furrowed his brow and fished out two banknotes from...
Chapter 627, "Definitely Not an Accident", is the clearest early answer because it turns Nina's situation into a concrete plot event. A useful checkpoint is the way the scene starts with He looked at the sinister smile lingering in Jiang Li's eyes, then at...
Chapter 809, "Specially Made for You", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, Listening to the cries from inside the house that sounded like a pig....