One possible plot could be that Hermione is exploring some dark and ancient magic. She discovers a spell that has been long forgotten and decides to test it out. Unbeknownst to her, the spell was actually a dangerous transmutation spell that was used by dark wizards to transform their enemies into trolls. When she casts it, she transforms into a troll. This would then force her to rely on her wits even more, as a troll, to find a way to communicate with her friends and reverse the spell.
Well, first of all, Harry and Ron would be extremely confused and worried. They would probably try to hide her from the other students and teachers at first. If she was found out, it could cause a big panic in Hogwarts. Some students might be scared and run away, while others might be curious. And Hermione, as a troll, would have to deal with her new form. She might find it difficult to move around in the castle, as trolls are much bigger and less agile than humans in the narrow corridors.
One reason could be that the troll is simply on a rampage and Hermione was in the wrong place at the wrong time. Trolls are not known for their intelligence, and it might have just stumbled upon Hermione while wandering the Hogwarts halls.
One possible plotline could be Hermione accidentally gets hit by a spell that causes the transformation. Her friends then have to figure out how to reverse it while she has to deal with the new way people perceive her.
It could also be that the situation was extremely dire. For example, the troll was about to destroy something very important, like a magical artifact that could cause great harm if it fell into the wrong hands, and Hermione saw killing the troll as the only way to prevent that.
I'm not aware of any particular 'hermione transforms into a bimbo fanfiction'. Fanfiction can cover a wide range of themes and ideas, but this might be a rather unique and perhaps not as common transformation in the Hermione -centric fanfiction world. You could try reaching out to fanfiction communities on social media platforms to ask for recommendations.
Well, there might be a fanfiction where Harry and Hermione find a baby troll and decide to raise it secretly. They face a lot of challenges, like hiding it from the other students and teachers, and also teaching it basic things. This kind of story shows their compassion and their ability to take on unexpected responsibilities.
Some fanfictions might focus on the relationship between Hermione and the idea of the troll in a more metaphorical sense. For instance, the troll could represent an obstacle she has to overcome, and the story could be about her personal growth while dealing with this 'troll' in her life. This type of fanfiction can be really interesting as it gives a new perspective on Hermione's character.
He could have different interactions with the Hogwarts teachers. Teachers like Professor Snape might treat him (as Hermione) with a bit more respect due to Hermione's academic achievements. Harry could also use this opportunity to learn more about the inner workings of Hogwarts from the perspective of a different student. He might discover new secrets in the library or during private study sessions with professors that he wouldn't have as Harry.
It could be very important. Her truth - telling might be crucial in saving other characters from danger. If they have the wrong idea about the troll, they could make bad decisions.