In some fanfictions, Ash's over - ambition might lead to his death. He could be so determined to become the very best Pokémon trainer that he takes on challenges that are far beyond his capabilities. He might enter a dangerous and uncharted area in the Pokemon XY world where there are unknown and extremely powerful Pokémon and trainers, and ultimately meet his end there. Also, a betrayal from someone he trusts, like a fellow trainer who is jealous of his success, could also result in his death.
One reason could be a powerful and evil Pokémon that he couldn't defeat. Maybe he encounters a super - strong legendary Pokémon that has been corrupted or is in a rage and it ends up fatally injuring him. Another possibility is that he sacrifices himself to save his friends or his Pokémon. For example, he might put himself in harm's way to protect Pikachu from a dangerous situation.