Perhaps there is a threat to Sinnoh that only Ash's old Pokémon can help solve. Some kind of ancient evil that has knowledge of Ash's past Pokémon battles. Ash and his old Pokémon then have to work together to defeat this threat. They might need to combine their old strategies with new ones they learn in Sinnoh to succeed. And along the way, Ash might meet some of his old friends who are also in Sinnoh and they help him in his mission.
It could be Pikachu. Since Pikachu has been with Ash from the very beginning and is his most iconic Pokémon. It's very likely that in Sinnoh fanfiction, Pikachu would be frequently used as it has a strong bond with Ash and is also very powerful.
Ash could develop his character in Sinnoh by becoming more strategic in his battles. Since Sinnoh has a variety of strong and unique Pokemon, he'd need to think more carefully. Misty could develop by becoming more independent. Maybe she takes on some solo adventures in Sinnoh, away from Ash for a bit, and this helps her grow as a trainer.
His first Sinnoh Pokémon could be a Chimchar. Imagine Ash finding Chimchar being bullied by some other Pokémon. Ash steps in and Chimchar decides to join him. Chimchar's energetic and mischievous nature would add a lot of fun to Ash's Sinnoh adventures as it grows and evolves into an Infernape.
Ash may develop his skills further in Sinnoh fanfictions. He could become more strategic in battles, learning from the different types of Pokémon and trainers in Sinnoh. May, on the other hand, might expand her knowledge of Pokémon contests, incorporating Sinnoh - style elements into her performances. Their experiences in Sinnoh help them grow both individually and as friends.
In some fanfictions, Ash and May might get involved in a mystery in Sinnoh. For example, strange events are occurring that are affecting Pokémon behavior. They work together to solve the mystery, with Ash using his battle skills and May her knowledge of Pokémon contests. As they dig deeper, they find that an ancient power from Sinnoh's past is being misused.
Typically, his old team could include Pikachu, of course. Then there might be Charizard, Bulbasaur, Squirtle, and others like Greninja. These Pokémon have been with Ash through different adventures in various regions.
Maybe Ash's old Pokémon have unique skills that are rare in Kalos. So, he uses them to gain an advantage in Pokémon contests or battles. His Charizard, for instance, could be a formidable opponent against Kalos's dragon - type Pokémon, and this could lead to some exciting rivalries and plot twists.
One plot idea could be Ash and Misty getting separated from their group in Sinnoh and having to rely on each other to survive in a strange, new area. They could encounter a rare and powerful Pokemon that neither of them has seen before. Another idea is that they discover an ancient mystery in Sinnoh related to a type of Pokemon evolution that was thought to be lost. This could lead them on a journey to uncover the truth and help a particular Pokemon evolve in a new way.
It could be about a revised or remade version of Ash's adventures in the Sinnoh region in a fan - created story. Maybe it features new storylines, character developments, or different interpretations of the original Sinnoh plot with Ash as the central character.