Well, perhaps Ash is transformed into a Pikachu. At first, he's really confused and has to get used to his new small body and electric powers. He teams up with his old Pokemon friends, but they don't immediately recognize him. As he tries to communicate his identity, he also has to deal with Team Rocket, who are now even more eager to catch this 'strange Pikachu'.
One possible plot could be that Ash is transformed into a rare and powerful Pokemon like Mewtwo. He then has to navigate the Pokemon world from a different perspective. He discovers new abilities and strengths he didn't know about before. And he uses his knowledge of being a trainer to communicate with other Pokemon and form alliances to face the challenges that come his way.
In this fanfiction, Ash could be transformed into a legendary Pokemon like Rayquaza. He would be in a completely different form with extraordinary powers. He might have to stop an evil force that is threatening the Pokemon world. Since he's now a Pokemon, he has to figure out how to use his new powers effectively, while also trying to find a way to transform back into his human form.
One possible plot could be that Ash is on a journey in a mysterious forest. He stumbles upon an ancient and powerful crystal. When he touches it, a strange energy envelops him and he starts to transform into Mew. As Mew, he has to adapt to his new form. He discovers he can fly and communicate with other Pokémon in a different way. He then uses his new abilities to help his Pokémon friends who are in trouble in a nearby cave, fighting off a group of powerful rogue Pokémon.
A new, powerful and evil Pokemon trainer could be the antagonist. This trainer might be jealous of Ash's new form and powers as a Pokemon and wants to capture him to use his powers for his own gain. He could set up various traps and battles to try and subdue Ash.
One possible plot could be that Ash is in a strange, enchanted forest. He somehow gets caught in a magical spell that starts to transform him into a Zoroark. As he changes, he begins to experience the world from a Zoroark's perspective. He discovers new powers and instincts, like the ability to create illusions to protect his friends from approaching danger in the forest. His Pokémon friends are initially confused but then realize it's still Ash inside.
One possible plot could be that Ash is hit by a strange new type of Poké - ball created by a mad scientist. This ball has the power to transform trainers into Pokémon. Ash wakes up to find himself in the body of Pikachu. At first, he's disoriented but then realizes he can communicate with his other Pokémon in a new way. He teams up with Pikachu (who is now acting a bit like Ash) and they go on an adventure to find the scientist and reverse the transformation.
One possible plot could be Ash being chosen by Arceus to save the world from a new, mysterious threat that even the Legendary Pokemon are powerless against. He has to travel to different regions and form bonds with various Legendary Pokemon along the way, using their combined powers to face the danger.
One idea could be that Korrina and Ash team up for a special Pokemon tournament. They train together, with Ash sharing his experiences from his previous travels and Korrina teaching Ash some new fighting - type Pokemon moves. Their Pokemon also form unique bonds during the training process.
A great plot might be that Ash accidentally stumbles upon a hidden realm where Legendary Pokemon live in seclusion. His presence disrupts their normal life, but instead of being angry, they are intrigued by this human. Ash shows them his love for Pokemon through his actions. As a result, one by one, the Legendary Pokemon start to follow him around. His harem grows, and with their combined powers and his leadership, they start to take on challenges like saving other Pokemon from a new and dangerous disease that is spreading and is said to be related to an ancient curse that only the Legendaries can help break.
One possible plot could be that Ash finds a Riolu as his starter pokemon. Riolu is initially very shy and distrustful of Ash. But as they go on adventures together, Riolu starts to open up. They face challenges like battling against Team Rocket who tries to steal Riolu for its potential power. Ash has to train Riolu not just in battle skills but also in trusting him completely. This journey of building trust and growing stronger together could be a great fanfiction plot.
In this fanfiction, if Ash turns into a pokemon, perhaps he becomes a Charmander. He could struggle with being in a different form. He might have to adapt to the life of a fire - type pokemon, facing challenges like being weak against water - type attacks and trying to find a way to change back while still having adventures with his friends, who now have to protect him in a different way.
Suppose Naruto transforms into an Eevee. Eevee has the potential to evolve into different forms, much like Naruto has the potential to grow and become stronger. Naruto could explore different evolution paths based on his experiences and the challenges he faces. His friendly and curious nature would fit well with Eevee's character. He could interact with other Pokemon in a unique way, using his ninja knowledge to find new ways to use Eevee's basic moves and perhaps even discover new, never - before - seen evolutions.