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 go on a new adventure together. Maybe they discover a new and rare Pokemon that no one has ever seen before.
Ash could discover a new form of an existing Alola Pokemon. This new form could be the result of some kind of special evolution due to the unique energy in the Alola region. His friends would be amazed, and they would all help him research and understand this new discovery.
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, they could meet at a Pokemon research center. There, they discover a new type of Pokemon evolution that has never been seen before. Ash and Dawn decide to study it together. Their different perspectives - Ash from a battle - focused view and Dawn from a more nurturing, contest - related view - make their study really interesting. They also meet some new friends along the way who help them in this discovery.
In a Draco and Hermione meet again fanfiction, they might start off with hostility. Draco, having grown up with certain prejudices, could initially be cold. However, over time, they could discover new sides of each other. Maybe they find themselves on the same side in a new magical crisis, forced to work together. As they do, they begin to respect each other's skills and intelligence. Hermione's vast knowledge and Draco's cunning could combine to solve the problem at hand. Eventually, this could lead to a grudging friendship or even something more, as they see past their old Hogwarts rivalries.
In this fanfiction, they may face a new villainous team in Unova. This team could be causing trouble for the local Pokemon and trainers. Ash and Dawn would have to use their combined skills to stop them. Ash's battle skills and Dawn's ability to think on her feet during Contests would be valuable assets.
In this fanfiction, Ash and Dawn might go on an adventure together. They could encounter rare Pokémon and have to work as a team to catch them.
They could participate in the Kalos League. Ash's determination and Dawn's strategic thinking would make them a formidable team. They might also discover new Pokemon evolutions specific to Kalos and use them to their advantage in battles.
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.