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.
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.
One idea could be that Ash's Charmander gets separated from him in a mysterious forest. It has to find its way back while facing various wild Pokémon and challenges. Along the way, it discovers a hidden power within itself.
The plot could involve Ash and Chimchar facing a series of challenges in a new Pokémon league. They might encounter a powerful villain who has a special device that can nullify Chimchar's fire attacks. Ash then has to find a way to train Chimchar to use its other skills, like its speed and agility, to defeat the villain's Pokémon.
One possibility is that Ash discovers a young Rayquaza at the start of his journey. Maybe it's injured and Ash nurses it back to health. This could lead to a strong bond between them, and Rayquaza could become his most powerful Pokémon, helping him in battles against tough opponents like Team Rocket or other powerful trainers.
One possible scenario in this fanfiction could be that Ash's journey takes a completely different turn with Latios as his starter. Latios, being a powerful and intelligent Pokémon, might help Ash quickly gain a reputation in the Pokémon world. They could form a unique bond, different from what Ash has had with his other Pokémon in the original series. Ash could use Latios' speed and psychic abilities to his advantage in battles, making him a formidable trainer right from the start.
One possible plot is that Treecko gets separated from its trainer early on. It then has to find its way back, facing many dangerous Pokemon on the way. During this journey, it learns new skills and becomes more independent.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. These platforms have a large collection of Pokemon fanfictions, and you can easily search for ones specifically about Ash and Treecko.
The plot could start with Ash getting Riolu as his starter Pokémon instead of Pikachu. Riolu might be a bit of a troublemaker at first, not fully understanding Ash's commands. But as they journey together, they start to understand each other better.
One possible plot could be that Ash, instead of starting with Pikachu, starts with a Gastly. Gastly could use its spooky abilities to help Ash in battles. For example, it could use its ability to turn invisible to surprise opponents. It might also play pranks on Ash's rivals, like making them think they're seeing ghosts in the forest where they're camping.
Well, one possible plot could be that Female Ash starts her journey with a Datini. Maybe she discovers Datini has some unique abilities that set them apart from other starters. They could face challenges in the first gym battle together, with Datini using its speed to dodge attacks and Female Ash coming up with creative strategies.