Well, he could discover new magical abilities or knowledge. Since Hogwarts has a vast curriculum, when Harry gets serious, he may delve deeper into subjects like Ancient Runes or Arithmancy, unlocking secrets that were previously overlooked. This could also lead to him making new friends who share his new - found academic interests. For example, he might become closer to Hermione in a different way, as they study more advanced topics together. He could also start to influence his classmates to take school more seriously as well, creating a positive academic environment in Gryffindor.
One possible plot could be that Zoro, turned into a girl, has to deal with the different reactions of his crewmates. Some might be shocked, some might tease him. And he has to learn to adapt to his new physical form while still maintaining his tough fighting spirit.
The change in Naruto's character. Usually, he's the go - lucky and sometimes clumsy ninja. But in these fanfics, when he gets serious, it's like seeing a whole new side of him. It makes the story more exciting as he approaches problems in a different way.
A third theme could be facing greater challenges. In these fanfictions, the challenges are so big that Luffy can't just rely on his normal, laid - back self. He has to be more serious to overcome them.
One great 'Harry gets serious' fanfiction is 'The Serious Side of Harry' on FanFiction.net. It shows Harry dealing with the weight of the prophecy in a very mature way.
A third theme could be self - discovery. As Natsu gets serious, he may also discover new aspects of himself. Maybe he realizes he has more power within him than he thought, or he discovers a different way of thinking about his role as a dragon slayer. This can lead to some interesting character arcs in the fanfiction.
He becomes more responsible. Instead of getting into as many impulsive adventures, he focuses on his studies and the duties that come with being a student at Hogwarts.
Well, perhaps an enemy uses a magic spell to turn Zoro into a baby as a way to weaken the Straw Hat Pirates. At first, the crew is in chaos trying to deal with a baby Zoro. But as time passes, they see that baby Zoro has a pure sense of determination. He still tries to train, albeit in a very clumsy and cute way. And in the end, they go on an adventure to find the antidote to turn him back, with lots of comical and touching moments along the way.