It could be that Ash has a special mission given to him by a Pokémon deity or an important figure in the Pokémon world. The mission is to bring a specific set of Pokémon to Alola for some ecological or cultural reason. Maybe there are certain Pokémon that can help balance the ecosystem in Alola, like Pokémon that can control overgrown plants or deal with an overpopulation of certain species.
He could bring some Johto Pokémon like Houndoom. Houndoom is a Dark - type Pokémon that would add a new element to Alola's Pokémon diversity. Its intimidating presence and strong battle capabilities would surely make it a popular addition among trainers in Alola.
It's possible that Greninja's return is part of a prophecy in the Alola region. The local people might see Greninja as a sign of great change or a hero. So Ash and Greninja have to work together not just to battle but also to fulfill this prophecy, which could involve saving the Alola region's natural balance or something similar.
A possible plot development could be that Gible evolves during their time in Kalos. This would give Ash a more powerful Pokemon to compete in Kalos gym battles. Another development could be that Gible discovers a new type of move or ability while in Kalos, which gives Ash an edge in battles.
Well, perhaps Greninja will encounter some new and strange Pokémon in Alola that it has never seen before. Ash could use this opportunity to train Greninja further, and they might participate in some Alola - specific Pokémon contests or battles together. Greninja's presence could also stir up some friendly rivalries among the local Pokémon and trainers, leading to exciting adventures and growth for both Ash and Greninja.
In such a fanfiction, perhaps Ash could be betrayed by his friends in Alola. Maybe they misunderstand his actions or are influenced by some false information. For example, they might think Ash is secretly working for an enemy, which leads to the betrayal.
Well, Ash and his Pokémon might get involved in local Alola festivals. His Pokémon could participate in Pokémon - themed contests or challenges during these festivals. Also, they might team up with local trainers and their Pokémon to protect Alola from any potential threats, like a group of wild Pokémon causing chaos in a village.
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.
Ash could discover a new form of an existing Alola Pokemon. This new form could be the result of some kind of special evolution due to the unique energy in the Alola region. His friends would be amazed, and they would all help him research and understand this new discovery.
In this kind of fanfiction, they might have new adventures. They could explore the unique Alola region together, encounter new Pokemon species, and bond even more deeply. Maybe Serena would learn new battle strategies from Ash, and they would participate in local Alola Pokemon contests and battles side by side.
Ash and Anabel could go on a Pokemon-catching spree in Alola. They might compete to see who can catch the rarest Pokemon first.