Well, the plot could start with a young mechanic who is always around cars. One day, while working on a very special vintage car, there's a strange energy surge. The next thing he knows, he is the car. He experiences life from a different perspective. He can't speak in the traditional sense but tries to convey messages through his headlights and horn. He discovers he has new senses, like being able to detect mechanical problems in other cars just by being near them. He goes on a journey to find a way to turn back into a human while having many encounters with different people and cars along the way.
A car thief could be the main character. After stealing a high - tech car, something goes wrong during the getaway and he turns into the car. Now he's stuck in this form, and the people he stole the car from are still after him. He has to use his new abilities as a car to evade capture while also trying to figure out how to reverse the transformation.
Maybe in the fanfiction, there's a scientific experiment gone wrong. A human scientist was trying to find a way to bridge the gap between humans and monsters. Sans, being a curious skeleton, volunteered to be a part of the experiment. However, instead of just creating a connection, the experiment transforms Sans into a human. As a human, he experiences new emotions more intensely, like hunger and fatigue, which he didn't really have as a skeleton. He then uses his new human form to try and convince other humans that monsters are not to be feared.
One possible plot could be Sonic accidentally getting hit by a strange energy beam during a battle with Dr. Eggman. This beam could have some sort of mutagenic effect that slowly starts to transform him into a human. As he turns into a human, he has to deal with the loss of his super speed at first. He then has to adapt to living as a human while still trying to protect his friends and the world from Eggman's new threats.
Well, one possible plot could be that Sans discovers a mysterious magic spell or artifact that gradually transforms him into a human. As he changes, he has to deal with new sensations like feeling pain and hunger in a more intense way. He also might struggle with how other characters in the Undertale world view him now that he's different.
One possible plot twist could be that Saphira, as a human, has to relearn all her dragon skills in a human form. For example, she might have to find new ways to use her strength or her senses which were so much more powerful as a dragon. Another twist could be that she has to hide her true identity from other humans who might fear or try to use her if they knew she was once a dragon.
Well, one possible plot could be that the alien is conducting some sort of experiment on Earth. It sees humans as interesting subjects and decides to transform a human into a cow as part of its study on different species' behaviors. Maybe it wants to see how the human mind copes with being in the body of a cow, like how it interacts with other cows and its reaction to the simple life of a cow.
Maybe in the 'koro sensei turns human fanfiction', he has to deal with his past as an assassin. As a human, he might have flashbacks of his former life and try to atone for his sins. He could start by teaching self - defense or martial arts to kids in a local community center. And he might also face danger from his former assassin comrades who don't like the idea of him becoming a 'good' human.
Pikachu might find it difficult to communicate at first. Since it was used to making 'Pika - pika' sounds, it would have to learn human languages. It could start with simple words and phrases, and as it becomes more proficient, be able to have complex conversations. For example, it might try to explain to Ash how strange it feels to have fingers instead of paws.
The human - turned - Nergigante discovers that they can communicate with other Nergigantes in a way no human ever could. However, they find out that the Nergigantes are planning an attack on a human settlement. This creates a moral dilemma for the transformed human. Do they side with their new kind or try to protect their former species?
A good plot could involve Inuyasha turning human at unexpected times. This creates a lot of chaos as his friends are used to his demon form. For instance, they might be in the middle of a battle with a powerful demon, and suddenly Inuyasha turns human. His friends then have to protect him while also trying to figure out how to turn him back. It could also be interesting if his human form has some sort of hidden power or connection that they discover during this time.