Well, it depends on what you consider 'good'. There are numerous fanfictions out there with Ash and Latios. Some might be more action - packed, with Ash and Latios teaming up to battle against powerful foes. Others could be more about the emotional connection between them, like Latios becoming Ash's trusted friend and confidant, helping him through tough times in his Pokémon - training journey.
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.
In an 'Ash turns into Latios' fanfiction, perhaps Ash is turned into Latios as a way to better understand the Pokémon he has befriended. As Latios, he might be able to communicate with other Pokémon on a deeper level. For example, he could learn the secrets of legendary Pokémon that have always been a mystery. His friends, like Pikachu and Misty, would initially be shocked but then have to figure out how to help him reverse the transformation or support him in this new form.
As I said before, FanFiction.net and Archive of Our Own are great places to start. They have a large collection of Pokémon fanfictions, and you should be able to find ones with Ash and Latios easily by using the search function.
In a fanfiction, Ash might have a special bond with Latios. Maybe Latios senses Ash's pure love for Pokemon and decides to team up with him. They could go on adventures, facing powerful foes together. Ash could learn new techniques from observing Latios in battles.
A possible plot could be centered around the mystery of Latios. Since it's a very special Pokémon, there could be ancient secrets associated with it. Ash, with Latios as his starter, would be on a quest to uncover these secrets. Along the way, they would face other trainers who are envious of Ash having such a powerful Pokémon. Ash would have to prove that he is a worthy trainer of Latios through his skills and the bond they share.
Through shared adventures. For instance, if they get lost in a mysterious forest together. Ash would rely on Latios or Latias's ability to sense their surroundings and they would grow closer as they face the challenges of finding their way out. It could be as simple as Ash sharing his food with them during this time, showing kindness.
The uniqueness of 'ash is a latios fanfiction' lies in the potential for new storylines. Ash, being a human in the normal Pokémon universe, becoming a Latios opens up a world of possibilities. For example, he could explore areas that are only accessible to Pokémon, or he could communicate with other Pokémon in ways he couldn't before. It also allows for different relationships to be explored, like how other Pokémon and humans would react to Ash's new form.
It could be a fan - made story where the character Ash is somehow transformed into a Latios. It might explore new adventures, powers, or relationships that come with this transformation.
A common theme is friendship. Ash is known for making strong bonds with his Pokémon, and with Latios it's no different. Another theme is adventure. They often go on quests or explore new places together in the fanfictions.