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.
There is a fanfiction where Hermione creates a new version of the Polyjuice Potion. This new potion has some unexpected side - effects when she tests it on herself. It gives her not only the physical appearance but also some of the magical abilities of the person she was trying to imitate. This leads to a whole new adventure as she tries to control these new powers.
In a Harry Potter polyjuice potion mishap fanfiction, the person who drinks the potion wrongly might start to exhibit the wrong personality traits. Let's say they were trying to transform into Hermione but due to the mishap, they start acting like Draco Malfoy instead. This would cause chaos as they would be doing things completely out of character for the person they were supposed to be imitating. They might start bullying other students or being mean to their friends, creating a lot of drama and confusion in the wizarding world.
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.
Ron could be the main character in a Harry Potter polyjuice potion mishap fanfiction. Ron is known for his sometimes hasty decisions. He might be so eager to prove himself or get involved in some secret mission that he doesn't follow the potion - making instructions properly. This could lead to a mishap where he transforms into an unexpected person, and then has to navigate through the wizarding world in this new form, trying to hide his identity from his friends and family while also trying to reverse the effects of the potion.
The Polyjuice Potion could create misunderstandings in their relationship. For example, if Ginny sees Harry seemingly flirting with someone who is actually him under the influence of the potion, she might get angry. On the other hand, if Harry sees Ginny acting strangely because she has been transformed by the potion, he could worry about her being under some sort of dark magic influence.
One way is to have lemons be part of the ingredients in a 'twisted' version of the Polyjuice Potion. Maybe a dark wizard creates a new version where lemons are added, and it has different effects on the transformation. For example, instead of a perfect physical transformation, it gives the user a lemon - scented aura or makes their skin turn a faint yellowish - green like a lemon peel.
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.