Well, it could be used to represent the end of a Pokemon's life cycle. For example, when a Pokemon faints in battle, it could be described as starting to turn to 'ash and dust' symbolically. Then, another aspect could be an old, abandoned Pokemon cemetery where trainers go to pay their respects to the Pokemon that have passed on, with the idea of 'ash to ash, dust to dust' being very prominent there.
The theme can be incorporated into a Pokemon fanfiction by having a dark force in the story that aims to turn the entire Pokemon world into 'ash and dust'. Ash and his friends must fight against this force. Along the way, they might encounter places that have already been destroyed and turned into desolate wastelands of 'ash and dust', which serves as a motivation for them to keep fighting to save the rest of the world.
One interesting Pokemon fanfiction could be about a post - apocalyptic Pokemon world where the concept of 'ash to ash, dust to dust' is literal. Maybe all the once - lush Pokemon habitats are reduced to ashes and dust, and trainers have to struggle to find the remaining Pokemon. Ash could be a central character in this story, trying to bring back the world to its former glory.
Another way is that the Pokemon could help with the costumes for the ballroom dance. Sewaddle could use its silk to create beautiful dresses or suits for Ash and Serena. And Charmander could use its fire to add some special effects to their performance, like creating a warm, glowing backdrop for their dance.
You can make Ash a central character with unique control abilities. For example, Ash can control the movement of ash particles to create illusions or barriers. This would add an interesting dynamic to the story.
Papa could be the one who initially discovers Manaphy when it is in trouble and then he calls Ash for help. As Ash arrives, Papa guides him on how to approach and handle Manaphy gently. Throughout the fanfiction, Papa acts as a mentor to Ash in his relationship with Manaphy, providing valuable insights and support.
Ash could be a Pokémon trainer with a different starting point in this fanfiction. Born in 1987, by 1997 he might have already made a name for himself in local Pokémon battles. He could have a team of Pokémon that he rescued from difficult situations, like a Charmander that was abandoned by its trainer. His story could revolve around giving these Pokémon a second chance and proving that Pokémon don't need to be 'perfect' to be strong.
In some fanfictions, Ash can touch Pokémon to show a deeper bond. It might be a way for the author to create more emotional and physical interactions between Ash and his Pokémon pals, different from the normal anime where physical contact like hugs is not as common.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
In the fanfiction, the fan girls may interact with Ash in a more collaborative way. They could join him on his travels for a short time. During this period, they help him with tasks like cooking for the pokemon or setting up camp. They also exchange stories about their experiences with pokemon, and the fan girls might learn a thing or two from Ash about being a better trainer.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.