A plotline might be that Latios is from a different dimension or time, and it has come to Ash's world for a specific purpose. Ash gets involved in this mission, and together they have to face powerful enemies. This could introduce new Pokémon species from other dimensions and new battle strategies that Ash has to learn to keep up with the challenges.
Maybe a mysterious organization that wants to experiment on Latios. They believe that Latios holds the key to some sort of genetic or power - enhancing technology. They send out their agents, who are skilled in Pokémon capture and combat, to try and get Latios. They don't care about Ash at first, but when he stands in their way, they see him as an obstacle to be removed.
One possible plot could be that Ash meets a Latios in a mysterious forest. Latios is injured, and Ash decides to help it. As he nurses it back to health, they form a strong bond. Ash then discovers that Latios has a special mission related to protecting a hidden power in the forest. With the help of his Pokémon, Ash joins Latios on this mission, facing various challenges along the way.
Well, I haven't really read a particular 'ash starts with latios fanfiction' myself. However, if you join some Pokémon - related fan groups on social media platforms, like Pokémon fan clubs on Facebook or Reddit communities dedicated to Pokémon fanfiction. You could ask the members there if they know any good ones. They are often very helpful in sharing their favorite fanfictions.
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.
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.
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.