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.
James Potter is often a popular choice. He's the leader of the Marauders, a talented wizard, and his relationship with Lily Evans is a major draw. His transformation from a bit of a troublemaker to a more responsible person is also an interesting aspect to explore.
Remus Lupin is highly popular too. His struggle with being a werewolf and his kind heart are aspects that fanfiction writers like to delve into. And of course, Peter Pettigrew, as the traitor, can be an interesting character to write about, exploring his motives and what could have been done to prevent his betrayal.
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.
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.
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.
Helga Hufflepuff is also quite popular. She is often portrayed as the warm - hearted and nurturing founder. In time travel fanfics, she can be a source of comfort and wisdom for the time - traveling characters, and her unique brand of magic might be explored more deeply.
Harry himself is obviously very popular. He's the main character in the original series, so in the 'Harry Potter Goes Muggle' fanfiction, readers are interested in seeing how he copes in the Muggle world. Hermione is also popular. Her intelligence and quick - wittedness make for interesting scenarios in the Muggle world, like helping Harry with Muggle studies.
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.
Harry Potter himself is very popular. Since he's the main character of the original series, many fanfics revolve around him, either continuing his story or changing certain aspects of it. Hermione Granger is also a favorite. Her intelligence and strong - willed nature make her a great character to explore further. Draco Malfoy is another popular one. Writers often like to redeem his character or explore his inner thoughts and struggles.
The main characters are Harry Potter for sure. He's the one who is reborn into the Marauders era. Then there are his parents, James and Lily. They are important as Harry gets to know them in a whole new way. The Marauders are also central. Sirius, with his wild and loyal nature, Remus with his werewolf secret, and Peter, who later betrays them in the original story. Their relationships with Harry add a lot of depth to the story.