Well, it could be about a magical spell. Maybe he offended a witch and she turned him into a girl as a punishment. He then has to go through his day trying to figure out how to reverse it while dealing with all the new experiences of being female, like wearing different clothes and having different social interactions.
It could also be a reincarnation - like story. He wakes up as a girl but still has memories of his past life as a guy. He has to adapt to this new life while also using his knowledge from his previous male existence. For example, he might use his knowledge of male - dominated fields in his new female body to break gender barriers in this new world he finds himself in.
Maybe in the story, the straight guy wakes up gay and suddenly finds himself attracted to a guy at his workplace. He then has to deal with the confusion and the potential reactions of his colleagues. It could involve him trying to figure out if it's just a passing phase or a real change in his sexuality. And depending on the choices in the interactive part, he could either explore this new side of himself or struggle to hold onto his old identity.
Maybe it's for a safety reason. The girl could be a rescuer in a make - believe scenario. The guy is 'trapped' in a dangerous place like a pretend collapsing building. So, the girl ties him up to keep him from moving around and getting more hurt while she figures out how to get him out completely. This would involve her being very careful and explaining to him what she's doing.
One possible plot could be that the guy wakes up in a girl's body and is completely shocked at first. He then has to navigate through the daily life of a girl, like dealing with different clothing styles, and the social expectations that come with being a female. Maybe he tries to find a way to reverse the situation while also learning more about the female experience.
One possible plot could be that the girl is a fellow student at Hogwarts who has fainted during a dangerous situation, like a duel gone wrong in a secret passage. Harry, being the heroic type, uses a spell to wake her up. Then they end up having to work together to get out of the passage before it closes or some other threat arrives.
In a more metaphorical sense, it might represent a character's sudden realization of having more masculine qualities or power. For example, she could be waking up to a newfound assertiveness or a sense of being in a male - dominated world where she has to adapt to male - like characteristics to survive or succeed.
One possibility is that he wakes up and discovers new allies. The story could then revolve around him building relationships with these new characters while still holding onto his old values. He may also have to deal with the fact that the world has moved on without him and find his new place in it.
Perhaps he wakes up to find that the laboratory is in the middle of a big experiment gone wrong. There are strange creatures or phenomena around him, and he has to figure out how to escape and also stop whatever bad thing is about to happen. He might use his Pokémon knowledge and skills to get through this tough situation.
In the fanfiction, when Ash wakes up in the room, he might discover that he has new, unexpected powers. The room could be a sort of testing ground for him. He'll have to learn to control these powers while also dealing with the mysteries of the room, like hidden doors and cryptic messages on the walls.
One reason could be self - defense. If the guy was harassing or threatening the girl, she might fight back to protect herself.
Maybe they are on a road trip, exploring new places on the motorcycle. Just the two of them, feeling the wind and freedom as they ride along the open roads.