You can try fanfiction platforms like Archive of Our Own (AO3). It has a vast collection of Pokémon fanfictions, and chances are you'll find some that involve Ash starting with Litten there. Just use the search function on the site.
One possible plot could be that Ash, instead of starting with Pikachu in the Alola region, starts with Litten. Litten could be a difficult Pokémon to train at first, but Ash's determination helps him bond with it. They could go on adventures in Alola, facing Team Skull together and Litten evolving as they overcome various challenges.
One popular 'Ash Catches Litten' fanfiction is 'The Adventure of Ash and Litten' on FanFiction.net. It focuses on their journey together and how Litten grows under Ash's care.
One great 'Ash starts with Eevee' fanfiction is 'Eevee's Adventure with Ash'. It shows Eevee evolving in unique ways under Ash's guidance. Another one could be 'Ash and Eevee: A New Bond' which focuses on their initial meeting and how they form a strong connection that helps them in battles.
Social media groups dedicated to Pokémon fanfiction can also be a great place to look. For example, on Facebook or Reddit, there are groups where fans post their works or recommend good fanfictions. So, search for relevant groups and look for the 'Ash starts with Greninja' fanfictions there.
In this fanfiction, Ash could develop better strategic thinking. Froakie's abilities would force Ash to come up with new battle plans. He'd study Froakie's moves and how they can be combined in different ways, thus enhancing his strategic skills. He may also become more empathetic towards Froakie as he discovers more about its nature and feelings, leading to a more well - rounded character.
Ash could become more patient. Since Togepi is a relatively weak and cute Pokémon at first, Ash has to be patient in training it. This would be a change from his sometimes impulsive nature in battles.
Ash's character would likely become more confident. Starting with a Charizard, which is a very powerful Pokémon, would give him an edge in battles. He would also become more patient as Charizard can be difficult to train at times. For example, when Charizard refuses to listen, Ash would have to find new ways to communicate and gain its respect.
Ash would likely become more responsible. Since Dratini is a relatively rare and potentially powerful Pokémon, he'd have to take extra care. This could make him think more before acting in battles and other situations.