So, I know I'm already doing something slightly similar to this with "Message in a Bottle"
But I was bored and reading a bunch of Amourshipping//Ash betrayal books because that apparently is a very consistent story type in the armourshipping community.

So I guess it would go something like this...
Ash Ketchum, the pride of Pallet, along with Gary Oak of course, has always thought he was an only child who had never known his father. But after that final move in the finale of the Kalos League, it seemed like a strange memory resurfaced in his mind. An explosion, darkness, the crackle of fire, screams. It was terrifying, right before his battling partner Greninja fainted to the Mega Charizard X. Thinking very little of it, Ash recovered from the devastating loss and returned to Kanto, only to have more and more things trigger visions that never matched up with his childhood memories. A quiet, kind man that resembled Ash himself putting him to sleep. Chasing another, barely older girl with raven hair like his around on a grassy lawn. It finally raised the questions; "what were these visions? Memories? And if they are memories, how come I only remember them now? And if so, then what and who are my family?"

IDK, just had a brainstorm moment. Still have to finish other stories first, lol.

Edit: I also had a second idea for a while that o totally forgot about XD
To sum it up, the world has ended. At least the way it used to be. Team Rocket has taken over Arceus's mind, using the god of Pokémon to destroy the world as we knew it. Ash Ketchum, a hero like no other, had attempted to stop and free Arceus. In the end, he failed, and once going through the wreckage, all that was left of the brave trainer and his team was a burnt hat. My name's Fire. I named myself, after my parents died, I assumed. Most people live in hidden bunkers or safehold bases, fighting or hiding from the Rocket Empire. I live alone, wandering what I think used to be Kanto. I'm going to stop this madness once and for all.