Ash ends up in the world of Date a Live, he also has all his pokemon, including his most recent, and first mythical pokemon, Meltan. They are stored in the portable storage box thing from pokemon let's go. Have Ash give his old pokemon some love, and also, I'm fairly sure that Ash has seen his old pokemon, just off screen. And let's have Ultra Beasts appear sometimes, Ash could have an irritation about Origamii and how she thinks about spirits, I know I do.