Another possibility is a trip to a far - off island. The island could be home to rare Pokémon that Ash has never seen before. Ash's mom could be taking him there to have a unique Pokémon - watching experience. They might also meet some local people on the island who have their own special ways of living with Pokémon.
It might be an adventure to an ancient ruin. Ash and his mom could be exploring the old, crumbling structures, looking for lost treasures. His mom might be teaching Ash about the history of the place as they search, and they could encounter some traps or puzzles along the way.
They could have many adventures. For example, they might explore the beautiful and diverse regions of Kalos. Latias could use its psychic powers to help Ash find rare Pokémon or hidden treasures. They could also encounter new Pokémon species that are unique to Kalos and form new friendships with local trainers.
Naruto could have all kinds of adventures. He might get involved in a fight against one of the Earth's villains, like Frieza if he somehow showed up. Naruto's Rasengan could be a powerful weapon against such foes. He could also explore different parts of the Earth, like the mountains or the oceans, which are very different from what he's used to in his own world.
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.
They could face off against some hidden enemies in Uzu. Maybe there are remnants of an old enemy of the Uzumaki clan who want to stop Naruto from learning about his heritage. Naruto and Kushina would then have to fight together, with Naruto learning new combat strategies from his mother.
They might go on a journey to discover the secrets of Itachi's past. Naruto could learn a lot from Itachi about the Uchiha clan and their powers. Maybe they'll encounter other powerful ninja clans along the way and have to fight or make alliances.
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.
Ash could meet new friends. The city is filled with different kinds of people. Maybe he'll meet a trainer with a completely different approach to Pokémon battles, which could inspire him to improve his own skills. He might also meet locals who can show him around and introduce him to the hidden gems of Altomare.
They could explore ancient ruins. Maybe they find a hidden cave filled with rare Pokémon. Ash's battle skills and Lillie's knowledge of Pokémon lore would come in handy.
One possible reason could be jealousy. Maybe Ash has achieved more in his Pokemon journey than his friends, and they can't handle it. Another reason might be some misunderstanding. For example, an enemy could trick Ash's friends into thinking Ash has done something bad. Or perhaps there's some sort of power struggle. If there's a powerful item or a rare Pokemon involved, and Ash is the main contender, his friends might be tempted to betray him for their own gain.
One possible plot could be that Serena's mom discovers Ash's true potential as a Pokemon trainer and decides to help him in his journey. Maybe she has some old Pokemon training techniques that she imparts to Ash, which helps him win a big tournament.