They might finally make peace. In many fanfictions, Dudley has grown and realized how wrongly he treated Harry in the past. So when they meet again, they could start to have a normal conversation, maybe even become friends or at least be civil to each other.
Perhaps Dudley could be in some sort of trouble and Harry, being the hero he is, helps him out. This would be a huge change from their childhood relationship. Harry has magic and skills that Dudley doesn't, and this could be a moment for Dudley to see Harry in a new light and for Harry to show his forgiving side.
They might finally have a real heart - to - heart conversation. Dudley could apologize for all the bullying in the past. Harry, being the forgiving person he is, would accept it. Maybe they'd end up becoming friends and sharing stories about their different lives since they parted ways.
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 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.
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.
Well, in some fanfictions, when they meet again, Sasuke and Sakura could go on an adventure together. They might face new enemies and in the process, rekindle their relationship. Sasuke could show his protective side for Sakura more than ever, and Sakura would be there to support Sasuke both emotionally and in battle.
They might have an epic battle to test Luffy's growth. Shanks could see how much stronger Luffy has become and be proud. Maybe Luffy will show off his new Haki abilities.
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.