The plot could involve Ash starting his journey in a unique way. Since he has a shiny Charmander, it might attract the attention of Team Rocket more than usual. They could constantly try to steal it, creating many action - packed chase scenes. Ash would have to train his Charmander to be stronger to fend off these threats. Along the way, he could meet other trainers with rare Pokémon who are both friends and rivals, all while aiming for Pokémon League championships.
It would change his journey a great deal. Firstly, having a shiny Charmander would make him stand out more. Other trainers would be more likely to challenge him, either to prove themselves against someone with a rare Pokémon or to try and get his Charmander. Also, trainers with more experience might offer to help Ash train his shiny Charmander, which could lead to new training techniques. In Pokémon contests, his shiny Charmander would be a show - stopper, giving him an edge in those aspects of his journey as well.
It's not very common. The Voltron crew is usually busy with the production and development of the official content. However, if they do come across it, it could be an interesting peek into the fans' minds.
Charmander's character can be developed by showing its growth in courage. For example, at the start, it might be afraid of big, intimidating Pokemon in the mystery dungeon. But as it faces more challenges, it gradually overcomes this fear. Also, its loyalty to its friends can be explored. If a friend gets captured, Charmander could go to great lengths to rescue them. This would show how much it values its relationships.
In this fanfiction, it could be that Ash has a really good strategy. He might first make Gyarados use up a lot of its energy by making it attack Charmander's illusions or decoys. Then, when Gyarados is tired, Charmander can step in with its full power. Charmander could also use the environment to its advantage. Maybe there are some flammable objects around that can be set on fire by Charmander, creating a more powerful and widespread fire attack that Gyarados can't avoid.
Perhaps Charmander lost its family. In the Pokémon world, family can be important for protection and learning. Without them, Charmander would be all by itself, facing the world alone. It would be a sad situation as it might not know how to fend for itself at first.
Well, one sad Charmander story could be that it was abandoned by its trainer. Maybe the trainer caught it on a whim, but then realized they didn't have the time or resources to take care of it. So, poor Charmander was left all alone in the wild, longing for the warmth and care it once had.