A great plotline in such fanfics is when Harry has to deal with the time - travel paradoxes. Since he's in the Marauders era, any change he makes could have a huge impact on his own future. He might be hesitant to do too much at first. Also, his relationships with the Marauders are fascinating. He could become like a mentor figure to them in some ways, teaching them things he learned from Dumbledore. And the whole idea of him being in a world where Voldemort is still rising but in a different stage is full of potential for exciting story development.
A great plot idea could be Harry getting involved in the Marauders' pranks but also using his experiences to guide them towards more responsible magic use. He could also be a key figure in uniting different groups in the wizarding world during that time. Since he has a different perspective, he might be able to bridge the gap between pure - bloods, half - bloods, and Muggle - borns much earlier, which could have a huge impact on the future and the fight against Voldemort.
Obviously, Harry himself is a very popular character in these fanfics. After all, he's the one who has traveled to the Marauders era. Then there are the Marauders - James, Sirius, Remus, and Peter. Fans love exploring their relationships and personalities in more depth. Lily Evans is also quite popular as she's Harry's mother. Her interactions with Harry and how she develops in this different scenario are often a big draw for readers.
Well, in some 'Harry goes to Marauders time' fanfictions, a great plotline is Harry trying to prevent certain tragedies. He might attempt to stop the betrayal of one of the Marauders, knowing how it will all end in his own time. Also, his relationship with Lily and James could be a major part of the story. He could be the one to give them some advice that changes their fate, like how to better protect themselves from Voldemort.
One interesting plotline could be Harry trying to prevent the death of his parents. He might use his knowledge from the future to change certain events. For example, he could try to stop Voldemort's rise to power earlier in the Marauders' time.
Well, in some fanfictions, there are plotlines about the Marauders getting involved in some sort of mystery at Hogwarts that is deeper than the normal school drama. For example, they might uncover a dark secret about one of the teachers or a hidden part of the castle. This could lead to them having to use their combined skills and friendship to solve the problem and keep the school safe.
One great 'Harry goes to the marauders era' fanfic is 'Time Traveler'. It has an interesting plot where Harry has to navigate the challenges of the marauders' time while trying not to change too much of the future. Another is 'Marauder's Mayhem' which focuses on Harry's interactions with the marauders and how he fits into their group dynamic. There's also 'Back in Time with Harry' which shows a more emotional side of Harry as he meets his parents' generation.
One interesting plotline could be how the Marauders deal with Harry's magic emerging at an early age. Since they were quite mischievous themselves, they might have some unique ways of guiding him. For example, Sirius could teach Harry how to transform into his animagus form earlier than in the original story.
Another theme is the fight against dark magic. During the Marauders era, Voldemort was rising to power, so fanfics may show how the Marauders and their allies were involved in the early battles against the Death Eaters. There's also the theme of growing up and facing the responsibilities that come with being a wizard in a dangerous time.
One interesting plotline could be the identity of the masked marauders. Are they related to the original Marauders? Maybe they are descendants with a new mission. Another could be their interactions with Harry and his friends. Do they help or hinder Harry in his fight against Voldemort? It could also be about the secrets they hold that are related to Hogwarts' past, which could be a major factor in the story.
Maybe Harry finds himself in a situation where he has to team up with the Marauders to fight a common enemy that was previously unknown in the original Potter story. This would create a whole new dynamic and show how different generations of Hogwarts students can work together.