A really interesting plot could be that an ancient Alola legend speaks of a powerful fire - type Pokémon that is very similar to Charizard. Ash and Charizard then embark on a journey to discover the truth behind this legend. Along the way, they meet different characters who have their own stories related to the legend, and in the end, they find out that Charizard has a special connection to the Alola region's history.
Perhaps there could be a plot where Charizard accidentally causes a wildfire in Alola. Ash then has to take responsibility and with the help of the local Pokémon and trainers, they work together to put out the fire and repair the damage. This would show the importance of environmental protection and how Ash and Charizard learn from their mistakes in a new region.
One possible fanfiction could be about Ash bringing his Charizard to Alola for a special battle tournament. Ash and Charizard would have to adapt to the new environment and battle styles in Alola. They might face off against some of the unique Alola Pokémon and trainers, with Charizard's powerful fire - type moves being a real asset but also a challenge as they try to avoid causing too much destruction in the beautiful Alola region.
Another idea is that May decides to study Pokémon in Alola for a research project. Ash joins her to help. They visit different islands in Alola and encounter unique Pokémon training styles. They also face a group of poachers who are trying to steal some of the Alola Pokémon. Ash and May must use their skills and the help of their Pokémon friends to stop the poachers and protect the Alola Pokémon.
The climate in Alola might affect Charizard. Since it's a tropical region, it could make Charizard a bit more adaptable to warmer and more humid conditions. Maybe its fire - type moves will become even more powerful because of the abundant sunlight in Alola.
A great plot could involve Ash competing in a local Alola tournament that has some really strange and unique rules. This would force him to think outside the box and use different battle strategies. He might also make some new friends or rivals during this tournament who have their own interesting stories and Pokémon.
Perhaps in the fanfiction, Ash and Serena could stumble upon a mysterious Alola Pokémon that is said to have the power to grant wishes. They then go on an adventure to find this Pokémon, while also exploring the beautiful and diverse regions of Alola. During their journey, they help local Pokémon trainers with their problems and learn more about the Alola culture.
One idea could be that Ash discovers a female Charizard that has been separated from its group. He decides to train it, but it has a very different personality from his regular Charizard. Maybe it's more stubborn and has unique battle techniques that Ash has to learn to work with.
They might have all kinds of adventures. For example, they could explore the unique Alola regions, discover new Pokémon species that are native to Alola. Charizard might also have some interesting interactions with the local Pokémon, like friendly battles or just sharing the same space. Ash could use Charizard to traverse difficult terrains in Alola, like the volcanic areas.
It could be that Ash believes Charizard's experience and strength can be useful in Alola. There might be some tough challenges in Alola that only Charizard can help him with. Also, Ash might want Charizard to experience a new environment and different Pokémon species.
One interesting plot could be that Charizard's mega evolution causes it to lose some of its memories. Ash then has to go on a journey to help Charizard regain those memories while also dealing with the challenges that come with having a mega - evolved Pokémon. For example, other trainers might be jealous and try to challenge Ash more often. Also, Charizard's new power might attract the attention of powerful Pokémon that want to test their strength against it.
One idea could be that Ash and his Pokémon team up with each of the girls in the Alola harem to compete in a special tournament. They train together, share their Pokémon training techniques, and during this process, their relationships deepen.