Obviously, Ash would be the main character. His Pikachu would also play a major role as always. Then there could be Professor Oak, who might help Ash understand his new identity. Also, Gary Oak could be involved, maybe as a rival who is initially skeptical of Ash's new - found status.
One interesting plot could be Ash discovering his true heritage as the'son of legends' and having to learn special powers related to it. For example, he might be able to communicate with legendary Pokemon in a unique way.
The plot might involve Ash being hunted by villains who want to use his connection to the legendary Pokemon. His friends in the fanfiction could be either helping him keep his secret or be in danger because of it. Also, his journey to become a Pokemon master could be intertwined with his search for his parent's true identity and purpose, which was perhaps hidden from him for his own safety.
Ash could potentially have the power to control the elements related to Pokemon types. For example, if he encounters a fire - type Pokemon in trouble, he could use his ability to manipulate fire to help it. Also, he might be able to teleport between different regions of the Pokemon world, which would be a very handy ability for his adventures.
It changes the story in many ways. Ash would likely have some latent powers. Maybe he could communicate with Pokemon on a deeper level without the need for a lot of training. His battles could be different too. Opponents might be more hesitant to face him once they learn of his relation to Arceus. And in terms of the plot, it could introduce a whole new arc about Ash uncovering his true identity and what it means to be the son of Arceus.
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.
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.
It could be Mew. Mew is known for its ability to transform and has a very mysterious nature. If Ash was Mew's son, he might inherit some of that mystery and the power to communicate with Pokemon in unique ways.
The impact of Ash's loss in the fanfiction could extend beyond just himself. His Pokémon might also feel demoralized. However, this could also be an opportunity for them to grow stronger together. Ash could start training more intensively, seek new training methods, or even try to catch new Pokémon to strengthen his team.
One popular type of Pokemon Ash and Pikachu fanfiction might be adventure - based. For example, Ash and Pikachu could embark on a journey to a new and mysterious region filled with undiscovered Pokemon species. They might face challenges like difficult terrains and powerful, previously - unseen Pokemon. In this fanfiction, Pikachu's electric powers could be enhanced in some unique way, and Ash would have to strategize differently to overcome the obstacles.