One interesting plotline could be Harry using his future knowledge to prevent certain bad events from the start. For example, he might stop Draco from becoming a bully right away by befriending him early. Another could be him helping Hagrid avoid getting into trouble with the illegal dragon. He could also use his experience to excel in all his classes from the very beginning and gain a different kind of reputation among the teachers and students.
Obviously, Harry himself is the central character. Hermione is also often involved as she is Harry's friend. Ron too, as they are a trio. Dumbledore usually plays an important role as he is the headmaster. Snape is also a popular character in these fanfictions as Harry might try to change their relationship. And of course, Hagrid, who is always there for Harry in the original story and could be involved in new and different ways in the fanfiction.
Well, he might try to change certain events. For example, he could prevent Cedric from entering the Triwizard Tournament, thus avoiding his tragic death. He might also use his knowledge of future events to outwit Voldemort earlier.
One challenge could be not being able to change things as easily as he thinks. Time might have its own rules. Another is that he might accidentally change something that has a much worse consequence later.
One interesting plotline could be Harry going back in time and preventing certain tragedies from happening in the wizarding world. For example, he might stop the death of his parents. Another could be him using his future knowledge to change the way Hogwarts is run, like improving the curriculum for Defense Against the Dark Arts.
Luna Lovegood is also a popular character to be paired with Harry in these fanfictions. Her unique perspective and Harry's new - found knowledge from going back in time could lead to some really interesting adventures. They could discover new magical creatures or secrets about the wizarding world.
Hermione is another common character. Since she is always so intelligent and resourceful, Harry might turn to her for help in understanding the implications of time travel. They could work together to change certain events, like preventing the release of some dark magic that they later had to face. And of course, Ron would also be around. His loyalty and friendship would be a source of support for Harry as he navigates this new timeline.
In fanfiction, if Harry goes back in time to the god, he might gain some extraordinary powers. Maybe the god would bestow upon him the ability to control magic in ways he never could before. It could also lead to him changing the course of history, for better or worse.
The main antagonist could be the Devil himself. He might see Harry as an intruder in his domain and try to keep him there forever. He could set up all kinds of traps and challenges for Harry to overcome.
Well, in some fanfictions, Harry goes back in time to fix his relationships. He might go back to be a better friend to Ron and Hermione from the start. Also, he could use his knowledge of future magical items to gain an edge. Like, he could get his hands on some powerful spells or artifacts before Voldemort does. And sometimes, he goes back to change the Hogwarts curriculum, making it more practical and useful for the coming battles.