Well, there's a fanfic where Ash starts with Treecko called 'Treecko's Ascent with Ash'. In this one, Treecko is shown to have unique abilities that Ash helps to develop further. It also delves into how their partnership affects the other Pokémon in Ash's team. There may also be a fanfic that has a more comical take on Ash starting with Treecko, like 'Ash and Treecko's Hilarious Happenings', where they get into all sorts of funny situations during their travels.
You can try fanfic websites like FanFiction.net or Archive of Our Own (AO3). These platforms have a large collection of Pokémon fanfics, so there's a good chance you'll find some 'Ash starts with Treecko' stories there. Just use the search function and type in relevant keywords.
The 'ash catches treecko fanfic' might focus on the backstory of the Treecko. Perhaps it was a lone Treecko that had been through a lot before Ash came along. The fanfic could describe how Ash sensed its potential and was determined to catch it. It could also include details about the Poké Ball used, the battle (if any) that preceded the catch, and the moment of capture which could be really exciting and full of anticipation.
The relationship can develop through shared experiences. For example, if they face a tough battle together and have to rely on each other's skills to win. Ash could learn about Treecko's unique needs and habits, like what kind of food it prefers or the best environment for it to rest. This shows his care for Treecko.
You could try looking on popular fanfic websites like FanFiction.net or Archive of Our Own. There are a lot of Pokémon - related fanfics on these platforms, so there's a good chance you'll find it there.
The plot could be that Ash's powerful starter helps him quickly become a top trainer in his region. He might breeze through gym battles with ease and attract the attention of powerful trainers and organizations.
Well, it could start with Ash being in a new region where Treecko are very rare. He stumbles upon a young Treecko that is being bullied by other Pokémon. Ash steps in to protect it, and Treecko, seeing Ash's kindness, decides to join him. Their journey together focuses on exploring the unique ecosystems of this new region. Treecko has to adapt to new battle styles and opponents. Ash and Treecko also befriend a group of local Pokémon trainers who help them understand the secrets of the region. In the end, they face a powerful trainer who has a team full of strong Pokémon, and they have to use their combined skills and the knowledge they've gained to win the battle.
There could be a fanfiction where Treecko evolves in an unexpected way while with Ash. For example, it might evolve during a battle against a powerful opponent, and this new form gives Ash an edge in his Pokemon journey. In this story, Ash and Treecko's bond also deepens as they face the difficulties together.
Well, in addition to the main ones I mentioned before, there could be some of the local Pokémon trainers that Ash meets on his first day in Alola. There might also be some of the Totem Pokémon that he has to face during his adventures. And who knows, maybe some characters from previous regions that Ash has visited might make a cameo appearance in the Alola fanfic. For example, Brock or Misty could show up for a short visit to see how Ash is doing in Alola.
Well, it could be about Ash arriving in Alola and making new friends. Maybe he discovers new Pokémon unique to Alola and has all kinds of adventures while trying to master the Alola - style Pokémon training.