Harry Potter himself is quite common. Writers like to put him in situations where he has to use the potion to transform for various reasons, like infiltrating the Death Eaters.
Luna Lovegood can also be a very creative character in relation to the polyjuice potion. Given her unique perspective on magic, she might discover that the potion has properties that others have overlooked. Maybe she finds that it can be used to communicate with plants or other inanimate objects while in the form of someone who has a special connection to them.
The Polyjuice Potion can be used to create a mystery. For example, a character wakes up in a different body due to the potion and has to figure out who did this to them and why. It adds an element of suspense.
One interesting plot twist could be when the polyjuice potion is accidentally modified. For example, instead of just changing a person's appearance, it also gives them some of the magical abilities of the person they are transforming into. This could lead to a character who is normally not very powerful suddenly having extraordinary skills, which would completely change the power dynamics in the story.
One idea could be a situation where a group of Hogwarts students accidentally use the Polyjuice Potion and swap identities in the middle of an important exam. It would lead to chaos as they struggle to answer questions in each other's forms.
Well, perhaps a character uses the potion to turn into a teacher to change grades in order to get a friend out of academic trouble. They end up having to teach a class while in the teacher's form and hilarity ensues as they struggle to keep up with the subject matter. It could also lead to some self - discovery for the character about the challenges of teaching.
Obviously Merlin himself could be affected. He's the one with the magic and might accidentally expose himself to the potion. Arthur is also a likely candidate. As the king, if he's affected by the love potion, it would have a huge impact on Camelot. Morgana could also be a character affected. If she were under the influence of a love potion, it could change her plans and her relationships with other characters. And Guinevere too, since she has important relationships in Camelot that could be altered by the potion.
Well, he might experience a whole new perspective. He could face different reactions from his friends. For example, the boys might treat him more delicately, while the girls might be more open and friendly as they'd see him as one of their own. Also, he could use this new form to his advantage in some secret missions, like getting closer to certain female characters who might be more trusting of another girl.
Well, it could be for Lucius to spy on Draco's friends or to manipulate situations in Draco's favor without revealing his own involvement. Authors come up with all kinds of reasons to make the story more interesting.