A plot could involve Ash starting his journey with a Deino. Deino being a rather unique and not so common Pokémon. They could face challenges in training it as it's a bit wild at first. Ash might have to find special ways to bond with it, like exploring caves together which is a natural habitat for Deino. And through their adventures, they meet new Pokémon and trainers, with Deino gradually evolving and becoming a stronger part of Ash's team.
In this fanfiction, Ash's character would develop in several ways. First, he'd become more empathetic towards Pokémon that are seen as difficult or 'bad tempered' like Deino. He'd have to understand Deino's needs and fears to build a strong bond. Also, his leadership skills would improve as he'd need to lead Deino in battles and on their adventures. He might also become more resourceful, finding new ways to keep Deino happy and healthy, like searching for specific types of food that Deino likes.
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.
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.
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.
May could be one of the main characters. She could join Ash on his journey and share her knowledge about Hoenn's Pokemon with him. Another could be Brock, still tagging along to offer his expertise in Pokemon care.