If Ash had a different starter like Bulbasaur in fanfiction, his team composition would change significantly. He might focus more on grass - type Pokemon to complement Bulbasaur's abilities. His travels through different regions could be affected as well. Maybe he would be more drawn to areas with more plant - life where Bulbasaur could thrive. Also, his interactions with other trainers having fire - type starters would be more interesting due to type advantages and disadvantages.
One possible fanfic could be about Ash starting his journey with Snivy. Maybe it focuses on their initial struggles to understand each other. Ash, being used to more energetic starters, might find Snivy's calm and sometimes aloof nature a bit difficult to deal with at first. But as they face various Pokemon battles together, they gradually build a strong bond.
If Ash had chosen Bulbasaur as his starter. His connection with nature could be even deeper. He might have explored more grass - type Pokémon habitats. In battles, he could use Bulbasaur's status - changing moves like Sleep Powder to gain an edge. And his rivals would need to adjust their strategies when facing a grass - type starter - led team. This would change the whole dynamic of his Pokémon journey.
If Ash chose a different starter, the story would change drastically. For instance, if he had a water - type starter, his travel routes might be different as he'd seek out areas with more water - type Pokémon to train and catch. His battles against Team Rocket would also be different. A water - type starter could easily put out their fire - type Pokémon like Arbok and Weezing. This would force Team Rocket to come up with new plans to steal Pikachu (if Pikachu was still with him).
In this fanfiction, perhaps Ash starts with a Eevee. Since Eevee can evolve into multiple forms, it would add a whole new dimension to his journey. He could face different challenges as he tries to decide which evolution path to take for Eevee, and it would also attract different kinds of Pokémon opponents.
In a Pokemon fanfiction with Ash and Mewtwo, it could start with Ash encountering Mewtwo in an unexpected place. Maybe in a hidden forest where Mewtwo is observing the wild Pokemon. Ash, being his curious self, tries to approach Mewtwo. Mewtwo, initially hostile, realizes Ash's pure intentions and they start an unlikely friendship. This could lead to many adventures where Ash and Mewtwo face various challenges together, like battling against a new, powerful and evil Pokemon organization.
Well, in the fanfic, Ash might take Snivy to a special training ground where Snivy discovers new ways to use its vines for both attack and defense. This could lead to Snivy evolving faster than normal. And as they travel, they meet other trainers with different starters, and there could be friendly competitions that further strengthen the bond between Ash and Snivy.
Another possibility is that an old rival of Ash's, who has always been jealous of his success, becomes the main antagonist in this fanfiction. When Ash chooses a different starter, this rival sees it as an opportunity to finally defeat him, thinking that Ash's new and untested starter will be his downfall.
In fanfiction, the relationship between Ash, Ho - Oh, and his starter can be very creative. Ash's starter might be a rare or unique one that has a connection to Ho - Oh's legend. Ho - Oh could see Ash's pure heart through his relationship with his starter and choose to guide them on their journey. This could lead to Ash uncovering secrets about the Pokemon world that are related to Ho - Oh and the starters' origins.