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.
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.
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.
The plot could be that Ash is in Kalos to participate in a special Pokémon tournament. Misty hears about it and decides to come support him. Charizard, of course, is Ash's main battling Pokémon. Along the way, they meet a new type of Pokémon that is causing trouble in the region. They have to figure out a way to stop it, with Charizard using its powerful attacks. And in the end, Ash wins the tournament with the help of Misty and Charizard.
In the 'ash wins the kalos league fanfic', it could start with Ash arriving in Kalos with a new determination. He could encounter rare Pokémon early on that he manages to catch and train specifically for the league. His bond with his Pokémon could be a major factor. His Pikachu might evolve some new power, or his other Pokémon like Greninja could reach a new level of synchronicity with him. During the actual league matches, he could face off against some really tough opponents who have their own unique Pokémon and battle styles, but Ash prevails through his quick thinking and the strong relationships he has with his 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.
A great plot twist could be that the Kalos champion turns out to be an impostor. Ash discovers this during his journey towards the championship. This would mean that all the battles leading up to the championship were based on a false premise. Ash then has to not only expose the impostor but also face the real champion in a series of unforeseen challenges. Also, there could be a twist where Ash's Pikachu loses its ability to use electricity temporarily, forcing Ash to rely on his other Pokémon in new and creative ways.
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.