One possible scenario could be that Pikachu is exposed to a strange type of energy. This energy mutates its genetic structure, slowly transforming it into a human form. At first, Pikachu might be confused and scared, not understanding its new body. But as time passes, it starts to learn about human ways, like walking on two legs and using its hands to pick things up.
The human would probably be extremely confused at first. Suddenly having a different body, with four legs and a tail, and seeing the world from a much lower perspective would be disorienting.
One way a human could 'turn into' Zelda fanfiction is through creative writing. If a person is deeply inspired by the Zelda universe, they can start by imagining themselves as a character within it. For example, they could write about waking up in Hyrule, with all its unique landscapes and creatures. They might describe their experiences, like facing off against Bokoblins or exploring ancient dungeons. By using vivid details and elements from the Zelda games, they can create a fanfiction that is centered around their 'human' self in the Zelda world.
One idea could be that Optimus Prime turns into a human mechanic. He uses his knowledge of machinery from his days as a Cybertronian to revolutionize car repair in a small town. People are amazed by his skills and he secretly uses his new form to keep an eye out for any Decepticon threats.
One idea could be that Jack Frost turns into a human to experience the warmth of human emotions. He might find himself in a small town during winter. As a human, he discovers the joy of a hot drink on a cold day, something he never truly understood as a spirit. He could also fall in love with a local girl who is kind - hearted and loves the winter. This newfound love makes him question whether he wants to return to his former state or stay human forever.