One possible plot could be that Ash finds Charmander much earlier than in the original series. They could form a really strong bond quickly and start training in a different way. Maybe they focus more on speed and stealth rather than the usual battling approach at first.
In this fanfiction scenario, Ash and Charmander could have an adventure in a volcanic area. Since Charmander is a fire - type, it would feel right at home there. They might discover a new type of Pokemon that is only found in volcanic regions. Also, Ash could participate in a local fire - type Pokemon tournament in this area. With Charmander's growth and training, they could potentially win the tournament and gain some rare items or Pokemon eggs as rewards.
His relationship with Pikachu might become more competitive at first. Since Ash already has a strong starter in Charmander, Pikachu might feel the need to prove itself more. But over time, they could learn to work together better, with Pikachu using its electric attacks to complement Charmander's fire attacks in battles.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Ash starts with Charmander' on their search bars and you might find some interesting stories.
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.
One possibility is that the Charmander was found in a special place, like an ancient ruin. It has powers that are not fully understood at first. Ash, being the determined trainer he is, decides to figure out how to use these powers to their full potential. As they journey together, they face challenges not only from other trainers but also from mysterious forces in the Pokemon world that are interested in the Charmander's power. This fanfiction could be full of adventures, growth, and the development of a strong friendship between Ash and his Charmander.
Ash's character development is quite significant. At first, he might be a bit over - eager and not fully understand Charmander's needs. But as they spend time together, he learns patience. He realizes that Charmander can't always be pushed to its limits right away. His confidence also builds. When they win battles together, he starts to believe in his own abilities more. And his sense of loyalty deepens as he would do anything to protect Charmander.
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.
In the 'pokemon ash becomes a charmander fanfiction', it could be that Ash witnesses a Charmander perform an incredibly heroic act. For example, saving a group of Pokemon from a dangerous situation. This act of selflessness and courage makes Ash look at Charmander in a whole new light. He starts to study Charmander's behavior, training techniques specific to Charmander, and even starts collecting Charmander - related items. He becomes so obsessed that he starts to model his whole Pokemon - training approach around what he has learned from Charmander.
Ash would likely be more confident from the start. Since he has a powerful Charmander, he might be more eager to take on stronger opponents. He would also have to learn how to handle a Pokemon with such great power, which could make him more responsible.