Perhaps a character accidentally transforms into the wrong Animagus form. Say Neville was supposed to transform into a small, unassuming animal like a mouse, but due to a spell mix - up or some magical interference, he turns into a large and clumsy hippogriff. He would then have to deal with not only being in the wrong form but also the chaos that such a large and not - so - subtle form would cause in Hogwarts, like knocking over furniture and scaring the other students.
It can create chaos. For instance, if Harry had an Animagus mishap right before a crucial battle. If he was supposed to transform into a stag but instead turned into a small, defenseless animal, it would put the whole battle plan in jeopardy. His friends would have to either protect him or find a way to reverse the mishap quickly.
One possible plot could be that Tonks accidentally creates a potion that makes her swap appearances with Harry. This would lead to a series of comical and confusing situations. For example, Tonks (in Harry's body) has to deal with Harry's friends not realizing it's her, and she has to figure out how to reverse the effect while also not getting Harry into trouble at Hogwarts. Meanwhile, Harry (in Tonks' body) has to navigate the world of the Order of the Phoenix and deal with Tonks' responsibilities.
One interesting aspect could be seeing Harry's new form as an Animagus. Maybe he turns into a unique animal that gives him different abilities and perspectives in the wizarding world.
One interesting plotline could be Harry discovering his animagus form earlier than in the original series and using it to secretly explore Hogwarts at night. He might find hidden passages or secrets that were previously unknown.
Dumbledore could be his mentor. He has vast knowledge and experience, and he would likely see the potential in Harry being an Animagus. He could guide Harry through the difficult process of learning to control his transformation and use his Animagus form for good.
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.
Another possibility is that he becomes a fox. Foxes are clever and sly, which are qualities that Harry has developed over the years. As a fox, he could use his wits to outsmart his opponents. He could also easily blend into the forest environment, which would be handy for hiding or sneaking around.
One interesting scenario could be Draco and Ron being Animagus in a Hogwarts setting. Maybe they are both turned into animals during a chaotic Potions class accident. Draco could be a sleek white ferret, which is a nod to his Slytherin side and his somewhat sneaky nature. Ron, on the other hand, might be a ginger fox, representing his Gryffindor courage and his slightly mischievous side. They could then have to navigate the castle in their animal forms, causing all sorts of mayhem as they try to find a way to turn back.
Well, perhaps in the fanfiction, Harry becomes an Animagus to better spy on the Death Eaters. His form could be a small, inconspicuous animal like a mouse or a sparrow. He uses this ability to sneak into their meetings and gather crucial information for the Order of the Phoenix. It could also lead to some humorous situations, like when Hermione almost steps on him not realizing it's Harry in his Animagus form.
One interesting idea could be a character who was always thought to be a normal human but is actually a reverse animagus, meaning they turn into a human from an animal form instead of the other way around. For example, a cat that can transform into a wizard. It adds a whole new layer of mystery and backstory to the character.