Well, one possibility is that Ash continues his Pokemon training in Kalos, facing new and stronger opponents. Maybe he teams up with old friends from Kalos to take on a new threat to the region.
One possibility is that Ash decides to stay in Kalos to study Pokémon evolution in depth. Kalos has some Pokémon with very interesting evolution methods. He could team up with local Pokémon researchers and have adventures while learning more about how different Pokémon evolve under various conditions. This would not only expand his knowledge but also strengthen his bond with his own Pokémon.
He could interact with Serena again. Since they have a history in Kalos, they might have new adventures together. Also, Clemont and Bonnie are likely characters he would meet up with. They could continue their friendship and maybe work on some new inventions together with Clemont, or have fun times with Bonnie and her love for Pokémon.
Well, it could be about Ash exploring the new region of Kalos, meeting new Pokémon and trainers. Maybe he'll participate in the Kalos Pokémon League and face some tough opponents with his Pikachu and new Kalos Pokémon he catches along the way.
The plot could be that Ash enters a Kalos - only Pokémon competition with Charizard. They face strong opponents with unique Kalos Pokémon. Charizard might learn new moves by observing the local Pokémon. For example, it could see a Delphox use a powerful fire - type move and try to adapt it to its own style.
Well, it could be about Ash going back to Kalos to compete in some new Pokemon battles. Maybe he wants to challenge new trainers or participate in a special tournament there.
Obviously, Ash himself. And his Kalos friends like Serena would probably be there. Clemont and Bonnie might also make an appearance as they were important parts of Ash's Kalos journey.
In an 'Ash Wins Kalos League' fanfiction, one possible plot could be that Ash finally masters his Mega Evolution techniques better than ever. He has a series of intense battles where he outsmarts his opponents. His bond with his Pokémon, especially Greninja, reaches new heights. They use their unique moves in creative ways to defeat strong adversaries and claim the victory in the Kalos League.
Well, a great plot might be that Ash and Misty are in Kalos on a research mission about Pokémon evolution. Misty is especially interested in how certain water - type Pokémon in Kalos evolve differently compared to other regions. They get separated during their exploration in a mysterious forest in Kalos. Ash has to find his way through the forest with his Pokémon while also looking for Misty. Meanwhile, Misty discovers an ancient water - type Pokémon that is thought to be extinct. In the end, they reunite and share their discoveries.
Well, it could be that Ash has trained with some rare and powerful Pokémon in the Kalos region that are not typically part of his main team. This new addition to his arsenal helps him win the league. Also, he could have received some secret training from a legendary Pokémon trainer in Kalos, which gives him the upper hand in all his battles.
Well, it might mean that Ash is bringing his powerful and experienced Charizard into the Kalos region for some epic battles. Maybe he wants to show off Charizard's strength against the unique Pokemon in Kalos.