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.
Another plot could revolve around the love interests of the Marauders. For instance, James Potter's pursuit of Lily Evans. You could show their early interactions, the misunderstandings, and how their relationship gradually developed. It could also include how his friends supported or sometimes complicated his efforts to win her over.
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.
In such fanfiction, a plot point could be Harry's relationship with Snape. Since he knows Snape's story from his own time, he might try to change their relationship for the better. Maybe he could help Snape deal with his feelings for Lily in a more positive way. Also, Harry's experience with dark magic threats in his time could make him more proactive in dealing with the early signs of Voldemort's rise in the Marauders era.
Well, you could have an OC who is a half - blood trying to keep their identity a secret. The Marauders could accidentally discover it and help the OC deal with the potential consequences. Or an OC who is a seer and has visions about the future of the wizarding world during that time, which could include the rise of Voldemort. The OC could then try to enlist the help of the Marauders to prevent it. Also, an OC with a time - traveling device that malfunctions and lands them in the Marauders era, and they have to find a way to get back while interacting with the Marauders and other characters.
Voldemort would still be a major antagonist. Even in the Marauders era, he was starting to rise to power. Also, the Death Eaters who were following him at that time could be antagonists. They would be causing trouble and trying to spread their pure - blood ideology.
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 idea could be the new marauders getting into trouble with a new set of magical artifacts that have been hidden in Hogwarts for centuries. They might discover a map that leads to these artifacts, much like the Marauder's Map. Another could be their interactions with the ghosts in Hogwarts in a more adventurous way, like trying to solve a mystery that the ghosts have been keeping for ages.
One idea could be exploring the relationships between the four founders before Hogwarts was fully established. For example, how did they meet and decide to build a school together? Another could be delving into the creation of the sorting hat and the initial sorting criteria. Maybe there was a big debate among the founders about it.
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.
One interesting aspect is the new dynamics in relationships. For example, if a male character is made female, their interactions with other characters, especially of the opposite gender in the original, change completely. It gives a whole new angle to the friendships and romances.