One possible storyline is that they meet while investigating a strange phenomenon in the Pokémon world. They join forces to figure out what's going on and end up uncovering a plot by an evil team. Another could be that Misty has become a Pokémon trainer herself and challenges Ash to a battle when they meet. Or they could meet at a Pokémon sanctuary where they both have the same goal of helping injured Pokémon.
They might go on a new adventure together. Maybe they'll discover a rare Pokémon that neither has seen before and team up to catch it, just like the old days.
In Kalos, one potential rival for Ash could be a local trainer who specializes in Fairy - type Pokémon. Fairy - type Pokémon can be quite tricky in battles, and this trainer has mastered the use of their unique abilities. For Misty, a rival could be a girl who is an expert in water - type Pokémon battles, just like Misty herself. This girl might have some rare water - type Pokémon from Kalos that Misty has never seen before.
In Ash and Misty relationship fanfiction, their relationship is often portrayed as a deep and evolving one. They start as friends on their Pokémon journey. Ash's determination and Misty's feisty nature create a great dynamic. They support each other in battles and through various challenges, and over time, a romantic undertone may develop as they grow closer and understand each other better.
Perhaps in the fanfic, Ash has achieved a lot in his Pokémon journey, and Misty has also become a more skilled Pokémon trainer. When they meet again, they start to compare their experiences. Ash could show Misty his new and powerful Pokémon, and Misty could introduce Ash to some new training techniques she has learned. And then they decide to train together for a while to improve their skills even more.
Fanfiction about Ash and Misty's romance can take many forms. It could be set in different regions they've visited in the Pokémon world. Maybe in a story, they get separated during a journey and the longing they feel for each other during that time becomes the catalyst for their romantic feelings to surface. Some fanfic authors also like to include other Pokémon characters to add more drama and humor to their relationship.
In fanfiction, when Ash and Serena meet again, they might share a warm embrace. Maybe Serena has new Pokémon skills to show Ash. They could also start a new adventure together, exploring uncharted regions and facing powerful new opponents.
Maybe a mysterious organization that is trying to capture all the Pokémon in the area where Ash and Misty meet again. They would be the antagonists as Ash and Misty would try to stop them. This organization might have some powerful Pokémon and devious plans that Ash and Misty have to figure out and defeat.