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.
Well, they could change their own relationships. Perhaps Hermione would be able to help her younger self be more confident. Ron might stop himself from making some of the silly mistakes he made in the past. And Harry could get to know his parents better. They could also influence the development of magic in Hogwarts. For instance, introduce some new spells or magical concepts they learned in their original time.
Harry might be more confident. Since he already knows what will happen, he won't be as scared or uncertain. Ron could be less jealous. He has seen how his jealousy got in the way in the past. Hermione would probably be more laid - back. She won't have to worry as much about grades and knowledge because she already has it all.
They might go back in time and accidentally change something that has a huge ripple effect on the future. So then they have to scramble to fix it before they return to their own time. They could also go back in time to learn some long - lost magic from an ancient wizard or witch and bring that knowledge back to their time to defeat a new dark force.
One possible adventure is that they go back to a time when a powerful dark wizard was rising to power. They could use their combined intelligence and magic to stop him. Hermione would come up with clever plans, Luna would provide her unique insights and Ginny would be the brave one to execute dangerous tasks. They might also encounter young versions of their own relatives or teachers, which would be a very strange and interesting experience.
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.
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 possible plotline could be that Harry and Severus go back in time to prevent certain tragedies from happening in the wizarding world. For example, they might try to stop Voldemort's rise to power much earlier. Another could be that they go back to change their own relationship. Maybe in the original timeline they were enemies, but now they have a chance to become allies or even friends.