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.
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 could be a Pikachu. Since Pikachu is often associated with Ash and could be considered a special kind of 'first' pokemon for him in a different take of the story.
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.
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.
Pikachu could be a great choice as it has been Ash's iconic Pokemon. It would create a strong connection between the daughter and her father's journey. Another option could be a Eevee. Eevee has the potential to evolve into different types, which would offer a lot of possibilities for her journey and battles.
Ash could become more responsible as he has a unique starter that might need special care or training. He'd have to learn new ways to bond with it, which would make him more empathetic towards Pokemon in general. For example, if the starter is shy, Ash would have to be patient and gentle, thus developing his patience.
Another possibility is a Torchic. Torchic is a fire - type, and it's small and cute but can grow into a very strong Combusken and then Blaziken. It would bring a lot of energy to Fem Ash's team and could be a great partner for her in the early stages of her Pokemon journey, especially when facing off against grass - type Pokemon.
If Ash had a powerful starter, the anime would likely be more action - intense from the start. His battles would be quicker and more decisive, as his starter could overpower many opponents easily. This might also lead to different story arcs, as stronger opponents would need to be introduced earlier to keep the challenge level up.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. These platforms have a wide variety of Ash starter Pokemon fanfictions. Just use the search bar and type in relevant keywords like 'Ash starter Pokemon' and you'll get a list of stories to choose from.