One interesting plotline could be Ash and his friends trying to outwit Hunter J's devious plans to capture rare Pokémon. Maybe Hunter J sets a trap for Pikachu, and Ash has to use his wits and the bond with his Pokémon to rescue Pikachu.
In some fanfictions, there might be a plot where Hunter J has a change of heart. For example, after seeing Ash's kindness towards Pokémon, she starts to question her own actions of hunting them. This could lead to an alliance between her and Ash against other more malicious Pokémon hunters.
Obviously, Ash is one of the main characters. He is the Pokémon trainer on his journey to become a Pokémon Master. Hunter J is also a main character, a notorious Pokémon hunter who often causes trouble for Ash and his friends.
One possible plot could be that Hunter J targets Ash for some unknown reason. Maybe she sees his unique bond with his Pokémon as a threat or something she wants to possess. Ash, being his usual brave self, has to outwit her while also protecting his Pokémon friends from her devious plans.
One idea could be a story where Pokemon Hunter J has a change of heart. Maybe she encounters a particularly powerful and kind Pokemon that makes her question her ways. For example, she tries to capture a Latios, but it shows her kindness and she starts to hesitate.
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.
A cool Pokemon Hunter J fanfic could focus on her facing off against a powerful trainer who is trying to stop her illegal activities. The battles between their Pokemon could be really intense, with the trainer using unique strategies. It could also show her slowly realizing that there might be more to life than just hunting Pokemon for profit.
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.