One possible interesting plotline could be Dark Ash's struggle with his new, darker side while still having memories of his former self. Maybe he has to face off against his old friends who don't recognize him at first.
Obviously, Dark Ash is one of the main characters. He's the center of the story with his transformed state. Serena might also be a main character as she could be trying to reach out to Ash or deal with the new Dark Ash.
One possible plotline could be Ash's discovery of his own latent legendary powers. Maybe he starts to exhibit abilities that are unique to legendary Pokemon, like controlling the weather or having an extraordinary bond with all types of Pokemon.
One great fanfiction could be 'Ash and Serena's Unseen Adventure'. It might explore their relationship further as they journey through a new region filled with unique Pokemon. Another could be 'Serena's Dream Come True with Ash', where Serena finally achieves her long - held dreams while Ash is right by her side supporting her. And there's also 'The Bond of Ash and Serena', which delves deep into how their bond grew over their adventures.
One interesting plotline could be Ash facing a new and extremely powerful legendary Pokemon that challenges his skills as a champion in ways he's never experienced before. This could lead to him having to develop new battle strategies.
One interesting plotline could be Ash becoming a super powerful trainer overnight in the fanfiction. Maybe he discovers a new type of Pokemon power that gives him an edge in battles.
One interesting plotline could be Mallow and Ash going on a special cooking adventure where they combine their skills. Mallow's cooking knowledge and Ash's love for Pokémon could lead them to discover a new Pokémon that is attracted to Mallow's special dishes. They might then have to figure out how to catch it while also dealing with some challenges from Team Rocket who want to steal the Pokémon for themselves.
In some 'legendary pokemon and ash fanfiction', there might be a plotline where Ash travels to a hidden realm where only legendary pokemon exist. Here, he has to prove himself worthy to the legendary pokemon in order to gain their help in his journey to become a pokemon master. This could involve facing various trials and challenges that test his skills, courage, and friendship with his pokemon.
Another plotline could be Ash getting involved in some sort of virtual Pokemon world within the fanfiction that has unique rules and challenges. He could discover new Pokemon species that are exclusive to this gamer - themed world. This would add a whole new dimension to his adventures and his relationship with his existing Pokemon team as they all adapt to this new environment.
One possible plotline could be Ash encountering a new and extremely powerful Pokemon that he has to train in a unique way to become even stronger. Maybe it's a rare legendary Pokemon that has a special bond with him.
One interesting plotline could be Ash facing new and extremely powerful Pokémon that he has to strategize in unique ways to defeat. Maybe he discovers a hidden Pokémon species that has been lost for centuries and has to figure out how to befriend it. Another could be Ash getting involved in a Pokémon tournament with a much higher level of competition than he's used to, which forces him to rapidly improve his skills.