Well, Harry Potter is the main character who travels back in time to the Marauders era. James Potter is a significant character as Harry's father. Sirius Black, with his charm and wild nature, is an important part of the story. Remus Lupin, the intelligent and kind werewolf, is another mainstay. Peter Pettigrew, though not always a likable character, is also part of the main cast. And there could be other characters like Severus Snape in some fanfics, as his relationship with Lily and the Marauders was complex and could be explored in the context of Harry being in that era.
One popular 'marauders era fanfic' is 'All the Young Dudes'. It offers a detailed and engaging look into the lives of the Marauders during their Hogwarts days. It has great character development and a really interesting plot.
Fanfiction.net is another place. Although the interface might not be as modern as AO3, it still has a good number of 'marauders era fanfic'. You just need to be a bit more patient when searching through the categories and filtering out the ones that don't interest you.
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 great 'Harry in Marauders era fanfic' is 'Time-Turner Tales'. It delves into how Harry's presence in the Marauders' time changes the dynamics. Another is 'Harry Among the Marauders' which focuses on his relationships with the group. And 'Marauder - Harry Connection' shows some unique adventures of Harry with the Marauders.
Another good one is 'The Marauders' by an anonymous author. It focuses a lot on their pranks and adventures. It has a great sense of humor that really captures the spirit of the Marauders. You'll find yourself laughing out loud at their antics and also feeling the bond between the friends.
One great 'oc marauders era fanfiction' is 'The Marauders' Daughter'. It has a really engaging plot where the main OC is the daughter of one of the Marauders. The story does a wonderful job of weaving in the original Marauders' story while also giving the OC a unique and interesting backstory.
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.
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.
I also like 'Marauder's Boy'. It focuses a lot on James Potter's growth and his relationships. It shows his transformation from a bit of a troublemaker to a more responsible person. It has some really interesting plot twists that keep you hooked throughout.