Perhaps there's also a sense of relief. They might have both been through some tough times alone, and reuniting means having someone to rely on again. Sawyer might be relieved to see Ash's familiar face, and Ash could feel the same way about Sawyer. They can now share their burdens and joys once more.
They might have an intense conversation about their past adventures. Maybe they'll share stories of the battles they've been in and the Pokémon they've caught.
A good fanfic of their reunion should have a strong emotional connection. Show Ash's excitement and Greninja's loyalty. Also, a great plot like a new threat they must face together.
Perhaps they reunite in a forest where there's a rare Pokémon sighting. They both remember their adventures together and how they used to face challenges. They might start reminiscing about the times they got lost in similar forests before. Then, they could join forces to catch the rare Pokémon. Ash would use his battle skills, and Dawn would use her knowledge of Pokémon behavior to come up with a plan.
They might have an epic battle against a common enemy together. Greninja could show off some new moves it has learned during their separation, and Ash could use his experience from other adventures to lead them to victory.
There might be a feeling of shock. Given that Ash has always been the main protagonist and has survived so many dangerous situations in the official Pokémon storylines, his sudden death in a fanfic would be unexpected. This shock could then lead to a deeper exploration of the story, as readers try to understand how and why his death occurred and what it means for the world of Pokémon within the fanfic.
Optimism could also be an emotion. They may believe that the future will bring more success and positive experiences for them as a group and as individuals.
Some characters might feel flattered. If the fanfiction portrays them in a heroic and positive light, especially if it emphasizes their good qualities like Edward's determination or Alphonse's kindness, they might secretly feel a bit of pride and be flattered by the author's view of them.
Their reunion could be filled with a rush of old memories. Maybe Ash would show Misty all the new Pokémon he's caught. And Misty would tell Ash about her adventures back at the gym. They might even decide to travel together again for a while.
Misty would also be there to help. She's tough and resourceful. She might use her water - type Pokemon to cool down any burns if Ash has them, or to create a barricade against any approaching danger. Professor Oak could also play a role. He has a lot of knowledge and resources. He might send over some special medicine or a flying - type Pokemon to quickly transport Ash to his lab for proper treatment.
Probably frustration. Dealing with a world not fully adapted to wheelchairs can be extremely frustrating.