One idea could be that during detention, Hermione discovers a hidden magical artifact in the classroom. This leads to an adventure where she has to figure out its purpose and keep it safe from those who might misuse it.
A possible plot could be that Hermione is given detention by a new and strict teacher who has some strange rules. She then uses her intelligence to find loopholes in these rules while also trying to improve the situation for other students who are also in detention.
A plot where Snape, after his supposed death, is brought back in a way and Hermione discovers him. She then has to help him regain his strength and memories. Along the way, they form an unlikely friendship and explore the secrets of magic that Snape had been researching before his death. It could involve Hermione seeing a different side of Snape than she ever expected.
One plot idea could be that Hermione, through some magical mishap, ends up in the Marvel universe and meets Steve Rogers. She could use her magic to help the Avengers in a battle against a powerful enemy that their technology can't handle. For example, she might use a spell to contain a powerful energy source that was about to explode and destroy a city.
Maybe in the fanfic, Snape gives Hermione detention for accidentally disrupting his class. But instead of a typical punishment, he assigns her to help him organize his personal library of dark arts books. As they work together, Hermione begins to understand Snape's complex past and Snape starts to see Hermione in a new light, beyond just being a know - it - all Gryffindor.
One plot idea could be that during detention, Snape discovers Hermione's hidden talent for potion-making that even he hadn't expected. He starts to see her in a new light, not just as an over - achieving Gryffindor. Another idea is that they get trapped in the detention room due to a spell gone wrong, and they have to work together to get out, which leads to a grudging respect between them. And perhaps Hermione finds an old diary in the detention room that has some secrets about Snape's past, which she uses to understand him better.
One idea could be a story where Hermione and Viktor meet again years after the Triwizard Tournament. Maybe Viktor has come to Hogwarts as a guest lecturer and he and Hermione rekindle their friendship, which then develops into a deeper relationship as they work together on a research project about magical creatures that are native to both Bulgaria and Britain.
One idea could be a story where baby Harry and Hermione are magically transported to a different time or place. They could be in a magical forest full of strange creatures that they have to befriend in order to find their way back. Another plot could involve them as babies having a special connection that allows them to communicate without speaking. This could lead to them solving small mysteries in Hogwarts even at a very young age. A third idea might be that they are found by a group of magical beings who don't know who they are and raise them with different magical traditions, and later they discover their true identities.
A plot could involve FemHarry having a special connection to an ancient magical artifact that Hermione helps her research. As they dig deeper, they find out it has the power to change the wizarding world. They then have to protect it from dark wizards who want to use it for evil. This would lead to a lot of action and suspense.
Well, a plot might be that Hermione is deaged due to a magical accident in the library. As a deaged Hermione, she has a different view on things. She might start to question the rules of Hogwarts more, as she doesn't have the same level of respect for them as her older self. Her friends, Harry and Ron, would have to deal with this new, more rebellious Hermione while also trying to figure out how to reverse the deaging spell. This could cause a lot of chaos in Hogwarts.
One plot idea could be Hermione helping Hagrid with a new magical creature he discovers. Maybe it's a creature that has unique abilities and Hermione uses her extensive knowledge from books to figure out how to take care of it.
One plot idea could be that Hermione, through some magical accident, ends up in the Asgardian realm. Thor then has to help her get back home while she uses her magic to solve various problems in Asgard, like dealing with a mysterious magical threat that the Asgardians' magic can't handle.