Maybe Ash starts with a Eevee. Since Eevee can evolve into multiple forms, it would open up a whole new range of storylines. He could be on a journey to discover which evolution path is best for his Eevee, and along the way, meet different trainers and Pokemon that influence his decision. Also, Eevee's cute and friendly nature could attract different types of Pokemon to join his team.
Another possibility is that Ash starts with a legendary Pokemon like Mew. This would be a very different start as Mew is extremely powerful and rare. Ash would have to deal with the attention that comes with having such a special Pokemon. He might be targeted by evil organizations right from the start, and would have to learn how to protect Mew while also trying to understand its mysterious powers.
One idea could be that Ash starts with a Pikachu that has a rare and powerful hidden ability. This could lead to different battle strategies and interactions with other trainers. For example, instead of the usual Pikachu that gradually grows in strength, this one could be super strong from the start, causing other trainers to be both intimidated and curious.
One interesting plot could be Ash making unique new Pokémon friends in Alola. For example, he might encounter a rare Pokémon that only exists in the Alola region and form a special bond with it. This could lead to new adventures and challenges as he tries to understand and train this Pokémon.
One idea could be Ash starting in Unova with a new set of Pokemon he catches right away. Maybe he meets a unique and rare Pokemon that no one has seen before in that region. Another could be that he has to face a new type of villainous team that has different motives compared to the ones in other regions.
One interesting idea could be Ash having unique hybrid Pokemon companions. For example, a Pikachu with some other type's features. Another could be Ash himself having some hybrid abilities, like being able to communicate with Pokemon in a new and enhanced way.
One idea could be that Ash discovers a new type of Pokemon that has unique abilities and is the key to solving a big mystery in the Pokemon world. For example, it might be able to communicate with humans in a way no other Pokemon can, and Ash has to learn how to work with it to stop an evil organization from using this Pokemon's power for bad.
One idea could be to explore Ash's first encounter with Pikachu from Pikachu's perspective. Maybe Pikachu was initially scared of Ash but then grew to trust him over time. Another idea is to write about Ash's first gym battle in a more detailed and dramatic way, like adding some internal thoughts of Ash during the battle.
You can make it unique by changing the setting. For instance, if Ash starts with a Bulbasaur in a post - apocalyptic world where most Pokemon are scarce. This would force him to find new ways to train and care for his Bulbasaur, and also deal with other survivors who might be jealous or desperate for his Pokemon. It would be a completely different dynamic from the original series.
Another idea is Ash starting with a Bulbasaur. His journey could focus more on nature and plant - type Pokemon evolution. He might get involved in more environmental - related plots, like helping sick plants in the forest with Bulbasaur's powers.
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.
Maybe in the fanfiction, Elesa's Pokemon get lost in an unknown region and Ash offers to help her find them. During their search, they encounter rare and powerful Pokemon, and they also discover a hidden mystery about the region that they are in. This adventure could strengthen their friendship and respect for each other.
A possible plot is that Ash is separated from his friends during a powerful storm. Alone, he stumbles upon an ancient Pokemon ruin. He then has to decipher the secrets within the ruin with the help of his Pokemon, all while trying to reunite with his friends. He might learn some long - forgotten Pokemon training techniques in the process.