There could be time - traveling dark wizards who oppose Harry and Hermione's attempts to fix things in the past. These wizards might have their own agendas for the past and present, and see Harry and Hermione as threats to their plans. They could use dark magic to try and stop them or even manipulate time against them.
One interesting plot could be that they go back in time to prevent a certain dark wizard from ever rising to power in the first place. They might use their knowledge of future events to stop key events that led to the wizard's ascension.
In many of these fanfictions, the Time Turner is often the key device for time travel. It allows them to go back in time for a set number of hours. However, some fanfictions introduce new magical artifacts or spells that can bend time in different ways, like a special potion or an ancient wand.
In some fanfics, time travel can strengthen their relationship. For example, if they travel back in time and face difficult situations together, they rely on each other more. This shared experience can make their bond deeper.
Hermione going back in time to stop Voldemort's ascension is a big task. She would need to study the past thoroughly. She might try to change the events that led to Voldemort creating his Horcruxes. For example, she could prevent him from getting the information about Horcruxes in the first place. She could also try to influence the people around young Tom Riddle, like his classmates and teachers, to not be swayed by his charm and instead see his darker side.
Bellatrix Lestrange could also be a common antagonist. She was extremely loyal to Voldemort and very powerful. If Harry and Hermione's actions in the past disrupt the Death Eaters' plans, she would surely oppose them.
Well, they could meet younger versions of themselves and their friends. It would be really interesting to see how they interact. They might also try to give some hints or advice to their younger selves, but they have to be careful not to disrupt the timeline too much. And they could discover secrets that were long forgotten in the present time.
The main characters are Harry and Hermione, of course. Harry, the Boy Who Lived, with his Gryffindor courage and his special connection to Voldemort. And Hermione, the brilliant and studious witch, always full of knowledge and quick thinking.
Well, they might change some important events. For example, they could prevent certain tragedies from happening in the wizarding world. Maybe they'd save some characters who died in the original timeline.
In time travel fanfiction, Hermione facing a boggart could be really interesting. Maybe the boggart takes the form of her failing in her time - travel - related tasks. For example, she might be afraid of causing a paradox that destroys the wizarding world she loves.