Maybe a vengeful Pokemon. If Gladion has accidentally wronged a particularly strong and vengeful Pokemon in the past, like harming its family or taking something precious from it. This Pokemon could seek revenge and be the one to kill Gladion in the fanfiction.
It could be a power - hungry Pokemon trainer. This trainer might see Gladion as a threat to their goal of dominating the Pokemon league or the entire Pokemon world. So, they plan and execute his killing.
A secret and evil organization within the Pokemon world could be the antagonist. For example, an organization that wants to use Pokemon for immoral and destructive purposes. Gladion might be in their way of achieving their goals, so they get rid of him. They might have powerful resources and trained assassins to carry out the deed.
Chapter 715, "Childhood Shadow", is the clearest early answer because it turns Once Xiao Shixu's situation into a concrete plot event.
Chapter 732, "Sister Xiao Wan, I Don't Understand", 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 Bai Lu, eavesdropping in the corner, marveled at Jiang Cheng's...
Chapter 558, "The Love-Struck, Flirty Wife Appears", is the clearest early answer because it turns Ai Xiaochun's situation into a concrete plot event. A useful checkpoint is the way the scene starts with These people's comments are just too unpleasant; where does the fish...
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Chapter 706, "How Did a Good Person Turn Out Like This?", is the clearest early answer because it turns Shang Shaojing's situation into a concrete plot event.
Chapter 789, "Son Toad", is the clearest early answer because it turns Gu Yu's situation into a concrete plot event. The chapter gives readers a real answer by opening on Shang Shaojing had given her an envelope containing a piece of paper...
Chapter 904, "Welcome Young Master and Young Madam!", is the clearest early answer because it turns Yu Fei's situation into a concrete plot event. Early on, Jiang Bufan and Jiang Buchen immediately ran up in surprise..
Chapter 666, "It Really is His Mom!", is the clearest early answer because it turns Tai Fan's situation into a concrete plot event. The chapter turns that into something concrete by moving from The crowd was shocked as they turned their attention to the unknown,...
Chapter 756, "Birthday Banquet", is the clearest early answer because it turns Sister's situation into a concrete plot event. The chapter gives readers a real answer by opening on On the day of the birthday celebration, the Jiang Family got up early.
Chapter 608, "Congratulations to Team Q for a Successful Escape!", is the clearest early answer because it turns Captain Ji's situation into a concrete plot event. The chapter gives readers a real answer by opening on Ji Yanbai narrowed his eyes, still marveling at the audacity of this...