Well, they could be transported to an alternate magical dimension where the rules of magic are completely different. In this new place, they are seen as the only hope to restore order. Harry's quick thinking in dangerous situations and Hermione's vast knowledge of magic would be put to the test. They'd have to learn the new magic fast and find a way to get back to their own dimension while also saving this new one.
One idea could be that Harry and Hermione get married and have a family in a world where Voldemort was defeated much earlier. Their children could inherit both their magical abilities and intelligence, and they might face new threats to the wizarding world that require their combined skills to overcome.
Another plot could be that the baby's existence somehow threatens the balance of power in the wizarding world. Some dark forces might want to use or harm the baby for their own gain. Harry and Hermione then have to fight against these forces, all while raising their baby in a world full of danger and intrigue.
One interesting idea could be a trial where magic and intelligence are pitted against each other in a way not seen in the original books. For example, a character might have to use their smarts to outwit a magical spell during the trial.
One idea could be a time - travel plot where Hermione and Malfoy are sent back in time together and have to rely on each other to get back, while Harry is somehow involved in the events that led to their time - travel. Another could be a story where they are all part of a secret magical project that forces them to work together despite their differences. Malfoy might have some unique knowledge that Hermione respects, and Harry could be the mediator between their clashing personalities.
One idea could be a time - travel plot where Hermione somehow gets sent back in time and has to deal with a younger Bellatrix. Maybe she tries to change Bellatrix's path to the dark side. Another could be a magical duel between Hermione and Bellatrix where Harry is caught in the middle, forced to choose which side to help. Or a plot where Bellatrix, under some sort of spell, has to team up with Hermione and Harry to fight a greater evil.
One smart idea could be exploring the magic system in greater depth. For example, a fanfiction might delve into how different spells interact with each other in ways not fully explored in the original books. This can add new layers to the story and make it more interesting.
A plot could be that the Death Star is actually powered by a piece of Voldemort's soul. Hermione discovers this through her research. She then teams up with Luke Skywalker to find a way to destroy the Death Star in a way that also destroys Voldemort's soul fragment. It would be a race against time as the Empire is planning to use the power of the Dark Lord's soul to conquer more planets.
One idea could be that Viktor Krum, still having feelings for Hermione, comes back to Hogwarts for a Quidditch event. Harry notices his lingering glances at Hermione. This causes some tension between Harry and Krum as Harry is very protective of his friend. Meanwhile, Hermione is caught in the middle, trying to focus on her studies but also dealing with the unspoken emotions.
One plot idea could be that Hermione and Ginny team up to play a prank on Harry, but it backfires in a hilarious way. For example, they try to make his hair turn purple but instead it starts changing colors every few seconds. Another idea is that the three of them go on a secret adventure outside of Hogwarts to find a lost magical artifact. They face various challenges and their friendship is tested. And perhaps a plot where Hermione discovers a new spell that accidentally causes some chaos between Ginny and Harry's relationship, and she has to find a way to fix it.
One idea could be that they go on a magical picnic in a hidden meadow in Hogwarts grounds. They use magic to create floating plates and self - serving dishes of their favorite foods. Another could be that they attend a wizarding concert together and discover a newfound love for a particular type of magical music.