Ash might develop a deeper understanding of Pokemon relationships in the fanfiction. He could observe how different trainers in the new setting interact with their Pokemon and apply those lessons to his own relationships. For instance, he might learn about a special bond that a trainer has with a Pokemon that allows them to communicate telepathically. This could inspire Ash to explore different ways of connecting with his own Pokemon, leading to his character growth.
He could have adventures exploring new regions in the Pokemon Horizons universe that haven't been shown in the main series. Maybe he discovers a hidden island full of rare and unique Pokemon.
Obviously, Ash is the most important character as the story revolves around his death and return. Pikachu is also crucial as his closest pokemon companion. His friends like Misty, Brock, and Serena would play significant roles as they would be affected by his death and overjoyed at his return.
The other trainers in Pallet Town might look up to him even more now. They could gather around him, asking for stories about his battles and the rare Pokémon he's encountered. It would be a great opportunity for Ash to pass on his experience.
In such a fanfiction, a common plot point could be the shock and grief among Ash's friends when he dies. Maybe Pikachu refuses to leave his side at first. When he returns, he might have some new powers or a different perspective on his pokemon journey.
Perhaps he'll share his new adventures and the knowledge he's gained from traveling different regions. He could inspire the new generation of Pokémon trainers in Pallet Town to aim for greater things.
One idea could be a story where the main character discovers a rare and powerful Pokemon that is thought to be extinct. This Pokemon could have unique abilities that change the way battles are fought in the Pokemon Horizons universe.
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.
There could be unfinished business between them. They might have a shared goal that they haven't achieved yet. Greninja's return to Ash in the fanfiction could be to finally complete that goal, whether it's winning a particular Pokemon tournament or saving a certain place from destruction.
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.