They might have battles against strong Sinnoh trainers. Ash could use Charizard to take on gym leaders like Volkner. Charizard's fire - type moves would be really effective against some of the electric - type Pokémon in Sinnoh.
Ash may face challenges from the Sinnoh Pokémon League regulations. Maybe there are some specific rules about using a non - native Pokémon like Charizard. Also, there could be trainers in Sinnoh who are not used to seeing a Charizard and they might target it during battles, putting extra pressure on Ash and Charizard.
It's possible that he received it as a gift from Professor Oak. Oak might have discovered a way to transfer Mega Stones across regions and sent the Charizardite to Ash in Sinnoh. Another option could be that Ash made a trade with another trainer. He traded one of his Pokémon for the Mega Stone, knowing the potential of Mega Charizard.
In some 'ash takes charizard to kalos fanfiction', the plot could revolve around Charizard getting sick or injured in Kalos due to the new environment. Ash then has to search for a cure or a way to heal Charizard. This could lead him to meet some kind of Pokemon healer in Kalos who teaches him new techniques to take care of Charizard. And during this time, Ash also has to deal with any threats that come their way while Charizard is recovering.
Well, it could mean that Ash has an extremely powerful ally in the Sinnoh region. Maybe he uses Mega Charizard to battle against powerful gym leaders or face off against the evil teams more effectively. It might also attract the attention of other trainers who are either in awe or jealous of his powerful Pokémon.
In this kind of fanfiction, Ash and Charizard might encounter new Pokémon in Unova that they've never seen before. Charizard could have some really cool battles against Unova - native Pokémon. Maybe it would also meet some new friends or rivals. And Ash could learn new training techniques from the trainers in Unova to further improve Charizard's skills.
In fanfiction, when Ash takes Charizard to Alola, they might have all kinds of adventures. Charizard could help Ash in battles against the unique Alola Pokémon. It could also be a chance for Charizard to interact with new Pokémon species and form new friendships or rivalries. Maybe it would even learn some new moves from the local Pokémon trainers or from the environment of Alola.
One popular 'ash takes charizard to kalos fanfiction' might be 'Ash's Kalos Adventure with Charizard'. In this type of fanfiction, Ash usually takes Charizard to Kalos to explore the new region, face new challenges, and encounter different Pokemon and trainers. It often includes exciting battles and heartwarming moments between Ash and his Charizard.
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.