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.
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.
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.
One potential rival could be a male trainer who has a more traditional view of Pokémon training and looks down on the fem Ash's different approach. He might constantly try to one - up her in battles.
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.
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.
Perhaps an ancient evil spirit that was sealed away long ago. The hatching of the legendary starter's egg is a sign that the spirit can break free. It wants to possess the starter to regain its full power and take over the Pokemon world. This would make for a very dark and menacing antagonist, and Ash would have to use all his skills and the power of his friendship to defeat it.
Obviously, Ash is one of the main characters. He is the trainer who gets the legendary starter. And of course, the legendary starter itself is also a main character. It has its own personality and abilities that play a big role in the story.
The main difference is the gender of Ash. In the original, Ash is male, but in this fanfiction, being female, she might have different ways of handling situations. For instance, she could be more empathetic towards certain Pokemon types.
One interesting aspect could be the unique perspective of a female Ash. It might bring new character dynamics to the Pokemon journey. For example, different interactions with the Pokemon and other characters like Brock and Misty. Maybe she'll have a different approach to battles and training.
One possible plot could be that Ash finds a Riolu as his starter pokemon. Riolu is initially very shy and distrustful of Ash. But as they go on adventures together, Riolu starts to open up. They face challenges like battling against Team Rocket who tries to steal Riolu for its potential power. Ash has to train Riolu not just in battle skills but also in trusting him completely. This journey of building trust and growing stronger together could be a great fanfiction plot.