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.
Sirius Black is also extremely popular. His rebellious nature, his family issues (being from the Black family but not agreeing with their pure - blood supremacist views), and his loyalty to his friends make him a fascinating character. Fanfiction often delves into his backstory and his adventures outside of Hogwarts as well.
Lily Evans is a key character too. She's not only the love interest of James but also a strong - willed witch in her own right. Her friendship with Severus Snape and how that changed over time is another aspect that fanfiction writers like to explore. Plus, her role in standing up against the dark forces even before the main events of the Harry Potter series make her a great focus in Marauders era fanfiction.
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.
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.
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.
Albus and Scorpius are extremely popular. Their friendship and the unique dynamic between them, with Albus being a Potter and Scorpius being a Malfoy, offers a lot of room for exploration. Writers can delve into their insecurities, how they support each other, and how they deal with the expectations placed on them because of their families.
Harry Potter is definitely one of the most popular. Since he is the main character in the original series, fans often like to give him a second chance in various scenarios. Hermione Granger is also popular as she is so intelligent and fans might want to see her make different choices with a second chance. Another popular character is Draco Malfoy. His complex character and his family's situation make him an interesting choice for a second - chance story.
The Marauders era is a time full of potential for different storylines. There are so many what - ifs. Fanfiction writers can play with the idea of different magical abilities the Marauders might have had, or different relationships they could have formed. It also has a nostalgic factor for fans who love the original series and want to revisit that magical world from a different perspective.
I really like 'The Marauders' by an unknown author. It focuses on their adventures and how they formed their tight - knit group. It has some really exciting plot twists, like how they dealt with certain dark magic threats in their school days. It's a fun read that keeps you on the edge of your seat.
Dudley can also be a fascinating character to write about. You could show his growth from a bully to someone who might start to understand Harry. Maybe he has a secret admiration for Harry's magical abilities.
Definitely Harry Potter himself. He is the main character in the original series, and in fanfiction, his relationship with the Weasleys can be explored in many ways, like his bond with Ron as his best friend and his relationship with the Weasley parents as a sort of surrogate family.
Hermione herself is always interesting. She has her intelligence, determination, and strong sense of justice. When she meets the Marauders, her reactions to their pranks and their more carefree attitudes would be great to explore. Also, Sirius Black. He has his own complex backstory, and seeing how he interacts with Hermione, a girl from the future, could be really engaging. He might see a bit of himself in her rebellious side.