Well, it could be that some dark force is controlling Arceus and making it cause chaos in the Pokémon world. Ash, being the hero that he is, feels responsible for stopping this destruction. He trains really hard and uses his unique bond with his own Pokémon to take on Arceus. Maybe he discovers a weakness in Arceus during their fight that has to do with the power of friendship and trust, which are things Ash is known for.
One possible plot could be that Ash discovers a threat to the Pokémon world that only Arceus can stop. But Arceus is somehow corrupted or misinformed. So Ash has to fight Arceus to snap it out of it and save the world.
In an 'Ash Fights Arceus' fanfiction, perhaps Ash is on a quest to prove himself as the greatest trainer. He hears that Arceus is the most powerful Pokémon and decides to challenge it. Along the way, he meets various Pokémon and trainers who either help or hinder him in his journey to face Arceus in an epic battle.
Chapter 687, "How Did She Die Again?", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter gives readers a real answer by opening on In the process of pursuing the Protagonist Group, she gradually came... and ending on I'm busy here;
Chapter 615, "River·Doraemon·Li", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. Early on, Everyone huddled close with their bowls in hand.. By the end, This wind turbine is what I exchanged for yesterday..
Chapter 823, "The Jiang Lake Swindler Who Sponges Off Others", is the clearest early answer because it turns Feng Shui's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The Yu Family estate is elegant, and even the names of the dishes...
Chapter 752, "The Sudden and Unforeseen Reversal", is the clearest early answer because it turns Fang Wenzhe's situation into a concrete plot event. A useful checkpoint is the way the scene starts with By what right do you arrest my son?!" Upon hearing the commotion, Mrs.
Chapter 602, "Did Our Ship Capsize Too?!", is the clearest early answer because it turns Jiang Mingchuan's situation into a concrete plot event. Early on, Hmph." Jiang Zhao Yuan grunted again, "Has anyone been bullying you?"....
Chapter 735, "How is it Him?!", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. It opens with Besides..." Yin Le hesitated, "Changqing originally didn't let me say...
Chapter 569, "Watch How My Sister Torments You!", is the clearest early answer because it turns Team's situation into a concrete plot event. The chapter gives readers a real answer by opening on Due to the unequal number of members on Team J and Team Q, the system...
Chapter 580, "Old Sir Gu: I command you to start a live stream!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with After seeing those comments, Ai Xiaochun was overjoyed.
Chapter 662, "No Mirror, Is the Urine Matte?", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. It opens with Ji Yanbai and Mu Ye exchanged a glance and then began searching their...
Chapter 661, "Uninvited Guest", is the clearest early answer because it turns Zhuola's situation into a concrete plot event. It opens with A moment later, Zhuola let out a cold laugh, her voice hoarse as she... and closes on To his right was a security guard, sound asleep, and to his left was....