In the fanfiction, when they meet again, it could be a very emotional moment. Ash might share all his experiences from his travels without Misty, and Misty could do the same. They might realize how much they've missed each other's company and decide to travel together for a while, strengthening their friendship even more.
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 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.
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 discover a new and rare Pokemon that no one has ever seen before.
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.
One possibility is that they meet during a Pokemon contest. Dawn is excited to compete, and Ash is there to support her. Along the way, they encounter some rare Pokemon that neither of them has seen before. This leads to a joint adventure to learn more about this new Pokemon. Their friendship deepens as they work together, and Ash might even get some inspiration from Dawn's contest skills for his own battles.
They might have a really emotional reunion. Percy could give Annabeth a big, warm hug and they'd start reminiscing about their past adventures together, like the time they were in the Labyrinth or on their quests.
They might immediately embrace and share all the adventures they had separately. Gendry could show Arya the new skills he has learned, like advanced blacksmithing techniques. And Arya might tell Gendry about the different places she has been and the people she has met on her journey.
They might have an intense emotional reunion. Edward could be overcome with joy and relief at seeing Bella again. Maybe he'll take her into his arms immediately and they'll just stare into each other's eyes, all the past feelings flooding back.
Well, perhaps in this fanfiction encounter, Rowle has changed. He could approach Hermione with an offer of truce or alliance. Maybe he has seen the error of his ways in the dark wizarding world. Hermione, being the intelligent and cautious witch she is, would be very skeptical at first but might listen to what he has to say.
They might have a warm embrace. Haku could show Chihiro all the new and beautiful parts of the spirit world that have emerged since she left. They could share stories of their time apart, with Haku telling Chihiro about how he has been protecting the spirit world and Chihiro sharing about her life in the human world.