Hermione is smart enough to know that going back in time to stop Voldemort's ascension requires a multi - pronged approach. She would not only focus on Tom Riddle directly but also on the wizarding society that allowed his ideas to fester. She could work on changing the pure - blood supremacist views that were prevalent. By doing so, Riddle might not have found such a receptive audience for his dark ideas. She could also try to protect the potential victims of Voldemort earlier, like the Potters, so that Voldemort's power wouldn't grow as it did in the original timeline.
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.
If Harry went back to save Sirius, he might first need to figure out the exact time and place of Sirius' encounter with Bellatrix. He could use the Time - Turner more carefully this time. Once there, he could try to prevent Sirius from following Bellatrix into the Department of Mysteries. Maybe he'd bring some backup like Hermione and Ron, but they'd have to be very discreet. This would change the whole course of the story as Sirius was a key figure in Harry's life, and his presence could have influenced Harry's decisions and actions in later events, like the fight against Voldemort.
A lot of these fanfictions play around with the idea of changing destiny. In many of them, when Harry and Voldemort meet during time travel, they end up having to work together to fix some sort of magical catastrophe that their future selves created. This often leads to some very strange and unexpected alliances between the two arch - enemies.
In many fanfictions, Harry discovers a hidden ability within himself to stop time. This could be related to his connection with some ancient magic. Regarding the consequences, it could put him in a moral dilemma. If he uses this power to gain an advantage in a wizard duel, it might be considered cheating. Moreover, the wizarding world's time - keepers, like the Department of Mysteries, might take notice and try to stop him.
One common theme is changing the past. Fans often write about Hermione using her knowledge to fix things that went wrong in the original story, like preventing certain deaths or stopping Voldemort earlier.
One popular theme is redemption. Some fanfics show Voldemort getting a chance to change his ways through time travel. For example, he might see the error of his future actions when he meets a young Harry. Another theme is the exploration of their relationship. Since they are such strong enemies, seeing how they interact in different time periods is fascinating.
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.
There are many Hermione time travel fanfics. In some, she travels back to her first year at Hogwarts. She could use her knowledge from the future to outwit Voldemort's plans even earlier. Maybe she'd share some of the secrets she learned later with Harry and Ron right away, making their fight against the Dark Arts more efficient.
One great 'Harry Potter x Hermione Time Travel Fanfiction' is 'A Second Chance at Life'. It has a really interesting plot where they go back in time and try to change some of the major events in the wizarding world. The way the author writes their relationship development is quite engaging.
In some Hermione time travel fanfics, she might use time - turners in a more extreme way than in the original books. She could end up in a different era altogether, like the time of the founders of Hogwarts. This would open up a whole new world of magic and mystery for her to explore.