Perhaps she has a role in helping Ash when he gets into a really tough situation. Let's say Ash is captured by an evil organization. His sister, using her knowledge of the local area and her own Pokemon team, devises a clever plan to rescue him. She might have to team up with some of Ash's friends and form an unlikely alliance to succeed.
One interesting storyline could be that Ash's sister discovers she has a special power related to Pokemon that she has to hide from the world. For example, she can communicate with Legendary Pokemon, and this ability attracts the wrong kind of attention from villains who want to use her for their own gain.
One interesting storyline could be Ash befriending Mewtwo in a more in - depth way. Maybe Mewtwo decides to travel with Ash and his team, and they face challenges together. Ash could help Mewtwo understand the true meaning of friendship and trust, which Mewtwo has always struggled with due to its creation and past experiences.
One interesting storyline could be that Ursula tries to steal Ash's Pikachu for her own collection. She might use her wits and some devious Pokemon to outsmart Ash. Another could be Ash and Ursula teaming up by accident to face a common enemy that threatens both of their goals. For example, a powerful and rogue Legendary Pokemon.
One interesting storyline could be Ash facing a new, extremely powerful and mysterious Pokemon that challenges his entire understanding of battling. It might force him to develop new strategies and bond more deeply with his existing Pokemon team.
A possible storyline is that Nami gets lost in a strange forest while looking for a particular water - based treasure related to Pokemon. Ash stumbles upon her and they start to journey together to find this treasure. As they go deeper into the forest, they encounter new and unknown Pokemon species. They have to use both their Pokemon battling skills and Nami's knowledge of maps and directions to overcome obstacles like tricky mazes and wild Pokemon attacks.
One interesting storyline could be Ash exploring a new region in the 'pokemon red' world and finding rare and powerful pokemon that were thought to be extinct. He might have to face off against a new group of villains who have a different agenda compared to the ones in the original games and anime.
One interesting storyline could be Ash and Astrid teaming up to compete in a special Pokemon tournament. They might face tough opponents and have to come up with unique strategies using their Pokemon's skills. Another could be a story where they explore a mysterious new region together, discovering new Pokemon species along the way. And they could also be involved in a plot where they have to save a group of endangered Pokemon from an evil organization.
One interesting storyline could be Ash and Hapu going on a journey to discover a rare and powerful Pokemon. Along the way, they face various challenges and have to use their combined skills to overcome them.
One interesting storyline could be Ash and Wicke teaming up to discover a new and rare Pokemon. They might face various challenges like rough terrains and other trainers trying to stop them. Ash's battle skills combined with Wicke's knowledge of Pokemon could make for an exciting adventure.
There could be a fanfiction where Ash gets lost in a strange part of the Pokemon world and Officer Jenny is the one who finds him. She then helps him get back on track, but along the way they discover a hidden Pokemon community. This community has some unique Pokemon and a different way of life. Ash is fascinated and wants to learn more, while Officer Jenny is more cautious. Their different outlooks create some interesting interactions as they deal with this new discovery.
One interesting storyline could be Ash forming a unique bond with a rare Alola Pokémon. Maybe he discovers a Pokémon that has special powers and they go on adventures together to help it control and understand those powers.