Yes, there is one where Ash and Pikachu are transformed into different elemental types. Ash becomes a fire - type like a mini - Charizard and Pikachu turns into a water - type. It was a very unique take. They had to adapt to their new forms and learn how to use their powers in different ways. For example, Pikachu, as a water - type, had to figure out how to battle with water - based attacks instead of electric ones, which was really fun to read.
There's a fanfic where Ash and Pikachu get accidentally transformed by a strange machine in a Pokémon lab. Their transformation is more like a fusion of different Pokémon features. They then go on an adventure to find a way to reverse it. Along the way, they meet other Pokémon who are both scared and intrigued by their new forms. The story focuses a lot on their friendship and how they support each other in this strange new state.