His team might gain some new Kalos Pokémon. For instance, he could catch a Fennekin which later evolves into Delphox. This new addition would bring in new moves and abilities. Maybe he would also trade one of his existing Pokémon for a unique Kalos Pokémon that has a special advantage in the Kalos battles.
In this fanfic, Ash's Pokémon team could experience a significant change. His Pokémon might start to communicate in a different way, more in tune with the Kalos region's Pokémon communication methods. This could lead to them evolving new abilities or forms. He could also find some rare and powerful Kalos Pokémon eggs and hatch them. These new Pokémon would then need to be trained quickly to be part of his championship - ready team. Additionally, some of his existing Pokémon could gain new Mega Evolutions that are exclusive to the Kalos region, giving his team a huge boost in power.
His team could include some rare and powerful Kalos Pokemon that he didn't have in the show. For example, a shiny Goodra or a Mega - evolved Absol.
One possible fanfic could be about Ash's journey to become the Kalos champion. It might start with his arrival in Kalos, full of excitement and determination. He meets new Pokémon and friends along the way. His battles are intense, with him using unique strategies to defeat his opponents and gradually climb the ranks to the championship.
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.
Well, first of all, the perception of Team Rocket in the Pokémon world would change drastically. People would be confused and worried about Ash leading them. In terms of Pokémon - related activities, Ash could introduce new training methods to Team Rocket members. This might make their captured Pokémon stronger and more difficult to defeat for other trainers. Also, Ash could use his influence to make Team Rocket more involved in Pokémon conservation in a strange way. For instance, he might steal Pokémon from trainers who are not taking proper care of them and then place them in better environments. This would create a whole new set of rules and power dynamics in the Pokémon world, as other organizations and trainers would have to adjust to this new Team Rocket.
One key element is the battles. Since it's about Ash as the Kalos champion, the battles need to be exciting and well - described. His Pokémon's moves, the strategies he uses, and the reactions of the opponents are all important. Another element is the relationships. His friendship with his Pokémon, his human friends in Kalos like Serena, Clemont, and Bonnie. And also his relationship with the Kalos region itself, how he explores and falls in love with the place.
Ash's relationship with his Pokémon might become more complex. He could start to question if his Pokémon would still be loyal to him if they knew about his connection to Giovanni. Some of his Pokémon might be wary at first, but over time, they would probably stay by his side because of their bond with Ash.
One possible plotline could be Ash facing off against his toughest opponents in Kalos. He might have to battle against the Elite Four, each with their unique Pokémon and battle styles. Another aspect could be his relationship with his Pokémon evolving, like Pikachu learning a new powerful move just for these crucial battles. And of course, his friendship with his Kalos - based friends like Serena could play a big role, with them cheering him on and providing support in different ways.
His Pokémon would probably be very confused at first. Pikachu, for example, might try to zap Ash back to his senses if he starts acting like a Team Rocket member. They would likely sense that something is wrong with Ash and would do their best to get him away from Team Rocket's influence.
In some fanfictions, Ash might have a completely different starting Pokémon team. For example, if he goes back far enough, he could start with Pokémon that were rare in his original time. His Pikachu might not be his first Pokémon. Also, he could have Pokémon from different regions right from the start. He might have a Sinnoh Pokémon like Piplup instead of a Kanto Pokémon as his initial partner.
His Pokémon might be really confused at first. They're used to Ash as a human, so seeing him as a Pikachu would be quite a shock. But some of them, like Pikachu himself, might feel a closer bond since Ash is now in a similar form.