"Is there a way we can make this whole coma theory darker?"
"Like you even have to ask."

After losing Kalos's Champion League, Ash attemp to hang himself but he failed at tying the knot result in his fall. He hit the deck after that fall and damage his brain. Pokemon Sun/Moon anime was the fantasy make up by him, where he no longer need to be the very best like no one ever was anymore and finally going to school like a normal kid.