Well, it could introduce a whole new plot. Maybe Ash in the fanfic could have new powers or go on different adventures in a unique Pokemon world created by the fanfic writer.
It could mean that the concept of 'ash', perhaps from something like the remains of a fire or volcanic ash, is being transformed into the basis for a new piece of fan - created fictional work. Maybe it's a metaphorical use of 'ash' to represent destruction or the end of something, which then gives birth to a new creative story within a fanfiction context.
Well, it could be about Ash somehow magically being transformed into a Latias. Maybe he then has to deal with the powers and instincts that come with being a Latias. He might try to find a way to turn back while also exploring the world from a new perspective as a Pokemon.
In 'ash turns into a gardevoir fanfic', the theme of identity crisis is quite common. Ash has been a human trainer for so long, and suddenly becoming a Gardevoir shakes his sense of self. He may question who he is now and where he belongs. Additionally, the theme of transformation itself is explored, not just the physical change but also the mental and emotional shifts that come with it. For instance, how does his perception of the Pokemon world change? Does he view his own Pokemon differently now? These are all aspects that are often delved into in such fanfics.
Well, perhaps in the fanfic, Ash turns into a girl because of a curse. As a girl, Ash could start to see the Pokemon world from a different perspective. Maybe she would form new relationships with her friends, like Pikachu being more protective. And she might have to enter Pokemon contests instead of battles at first due to the new appearance, but then find a way to get back into battles and show that she's still the same strong trainer inside.
A theme that often comes up is the reaction of others. His friends, like Pikachu and Misty, might be shocked or scared at first when they see Ash as Mewtwo. Also, there's the theme of adaptation. Ash has to adapt to his new form, which may be very different physically and mentally from his original self. For example, he might have to get used to Mewtwo's telepathic abilities and the way he moves.
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.
Maybe the main antagonist is an evil scientist. The scientist has been studying Pokémon transformations and when he sees Ash turn into Pikachu, he wants to experiment on him. He chases Ash around, trying to capture him with all kinds of high - tech devices. This forces Ash to use his Pikachu form's abilities and his human knowledge to outwit the scientist.
There could be a mad scientist - type character who created the situation where Ash turned into a Latias in the first place. But then, this character wants to keep Ash in that form to study him and use his new - found Pokemon abilities for their own gain. They would be the main obstacle for Ash in his journey to turn back into a human.
One possible plot could be that Ash turns into Pikachu and has to learn how to communicate with his human friends in this new form. He might struggle at first, but then discovers that he can use his Pikachu abilities in unique ways to help the group in battles and in solving problems around them.