In some fanfictions, Harry and Andromeda time travel together. They might go back to a crucial moment in the history of Hogwarts, like the founding of the school. There, they could interact with the founders and gain new knowledge about magic that they bring back to their own time to use against Voldemort or other dark forces.
One interesting plot could be Harry traveling back in time and meeting a younger Andromeda. He might end up changing some small events in her past that have a big impact on the future of the wizarding world. For example, he could prevent some minor tragedies in her family, which then alters the relationships between different pure - blood families.
One interesting plot could be the initial shock and confusion among the wizarding community. Harry is often seen as a hero and his sudden marriage contract with Andromeda would stir up a lot of gossip. Maybe the story could explore how they first react to this contract, whether they resist it or start to find some common ground.
One interesting plot could be that Harry and Amelia travel back in time to the founding of Hogwarts. They get to meet the four founders and witness the creation of the school's magical protections. Another plot might be them traveling to a time when a great dark wizard was at the height of his power and they have to find a way to stop him without changing too much of the future.
One interesting plot could be that Harry and Hermione travel back in time to change a major event at Hogwarts, like preventing the opening of the Chamber of Secrets. They might use their knowledge from the future to outwit Tom Riddle before he becomes Voldemort.
Another plot might involve Voldemort traveling to the future to see the destruction he causes. This could lead to a moment of self - reflection for him. He might realize the futility of his actions and perhaps change his ways. It would be a fascinating exploration of his character.
One interesting plot could be a character going back in time to prevent Voldemort's first rise to power. They could try to change the events that led to his initial gathering of followers. Another plot might involve a time - traveling student who accidentally alters the sorting of the houses, causing chaos and new alliances within Hogwarts. And a third could be a time traveler going back to the Marauders' era and getting involved in their pranks and adventures, which in turn has unforeseen consequences for the future.
One interesting plot could be the characters going back in time to prevent certain dark magic events from ever happening. For example, they might stop Voldemort's rise to power much earlier in history.
A possible plot is that Harry travels back to a time when the Marauders were in school. He could become friends with them and try to prevent some of the bad things that happened to them later. He could also use his knowledge of the future to help them in their pranks and adventures. This would be really interesting as he would be interacting with his father and his friends in a whole new way. It could also lead to changes in the future relationships and events at Hogwarts.
One interesting plot could be that Harry, Ginny and Teddy travel back to the time when the Marauders were at Hogwarts. They could interact with James, Sirius, Remus and Peter, and maybe even change some small events that have a big impact later. For example, they could prevent Peter from becoming a Death Eater spy, which would change the course of the war against Voldemort.
A cool plot in a smart Harry Potter time travel fanfiction could be that Harry travels back to the time when the Marauders were at Hogwarts. He could join their group and influence their actions in a positive way. Maybe he could prevent Sirius from being framed. Or, he could go back to the Triwizard Tournament time and figure out a way to expose Barty Crouch Jr. much earlier. Also, he might travel back to the time of the Goblin Rebellions and learn about the true relationship between wizards and goblins, which could change how he views things in his own time.
One interesting plot could be that Harry accidentally travels back to the time when his parents were at Hogwarts. He gets to see them in their youth, and maybe even interacts with them in a way that doesn't disrupt the timeline too much. For example, he could help his father with a prank that was a big part of his father's Hogwarts story.