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.
Well, it's possible that Ash and Mewtwo team up in the fanfic. They could face a common enemy together. Mewtwo's incredible psychic powers combined with Ash's experience and determination would make them a formidable duo. And perhaps this encounter would change Ash's view on some things, like the true nature of Pokémon battles.
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.
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 the fanfic, Ash and Misty might explore the Unova region's beautiful landscapes. They could come across a hidden lake where there are rare water - type Pokemon. Misty would be excited to study and catch them if possible. Ash would be there to support her and also learn more about water - type Pokemon from her.
Well, in the 'Ash and Misty joins Team Rocket fanfic', they could be brainwashed by Team Rocket's technology or some devious plan. Once they join, they might be given new uniforms and tasks. They could be involved in stealing Pikachu again, but this time from the other side. It would create a whole new dynamic in the Pokémon world, with their old allies now becoming their enemies.
In some fanfics, Misty might be portrayed as betraying Ash for power. For example, if there is a powerful trainer who offers Misty a chance to become an even greater trainer, but at the cost of leaving Ash behind. She could be tempted by the allure of advanced training techniques or rare Pokémon that this new opportunity presents.
Maybe they will have a friendly conversation, reminiscing about the past. Percy could share his new adventures, and Calypso might tell him how she has been since they last met.
Maybe Harry finally stands up to Aunt Marge in a more assertive way this time. He could use his magic subtly to teach her a lesson without getting into too much trouble.
Well, they might have a friendly chat about their past adventures. Maybe they'll share stories of the Pokémon they caught and the battles they fought together.
In this fanfiction, Ash might introduce his unique Pokémon skills to the Avengers. Maybe he could use his Pokémon to assist in battles against powerful villains that the Avengers are facing.