One common theme is the exploration of the characters' relationships. For example, many fanfictions focus on the strange friendship between Billy, Mandy and Grim. Another theme is the expansion of the show's dark and spooky universe. Writers often create new monsters or realms. Also, there's the theme of self - discovery, like when Billy or Mandy find out something new about themselves.
One great piece of 'the Grim Adventures of Billy and Mandy' fanfiction is 'The Other Side of Grim'. It delves into Grim's backstory in a really creative way, showing his life before he got stuck with Billy and Mandy. Another one is 'Billy's Big Adventure', which focuses on Billy having an unexpected journey through different dimensions. And there's also 'Mandy's New Rule', where Mandy decides to change the rules of the underworld and chaos ensues.
Obviously, Billy, Mandy and Grim are the most popular. Billy's stupidity and naivete provide a lot of comedic elements. Mandy, with her cold and calculating nature, is always interesting to write about as she often takes charge. Grim, being the Grim Reaper, has a lot of dark and mysterious backstory that can be explored. However, Irwin is also a popular choice as he is the vampire kid and his interactions with Billy and the others can lead to some really fun storylines.
One idea could be to explore what would happen if Grim lost his powers for a while. Billy and Mandy would have to figure out how to deal with the supernatural chaos without Grim's direct help. Another could be a time - travel plot where they go back to when Grim first became the Grim Reaper and try to change some of the rules of the underworld. Maybe they could also meet some of Grim's old friends or enemies from his past that would add a new layer of adventure.
Characterization is crucial in popular 'Mandy x Grim' fanfiction. Writers usually make Mandy even more headstrong and Grim even more otherworldly. The plot often has a bit of a dark or edgy tone, to match their personalities. For example, they might be on a quest to save the world from some dark force, but in a way that's different from the original story. There's also often a sense of humor, despite the dark themes. This can come from Mandy's sassy remarks or Grim's cluelessness about human things, which makes the story engaging and popular among fans.
One great Grim x Mandy fanfiction could be 'Grim and Mandy's Unlikely Alliance'. It might explore how their different natures, Grim being a reaper and Mandy having her dark and fearless side, come together in a series of adventures. Maybe they team up to face a greater evil in the underworld that threatens both the world of the living and the dead.
Fanfiction.net is also a great place to look. There are many different categories and fandoms represented on this site. Look for the Grim and Mandy section within the appropriate genre (probably something in the horror or supernatural genre) to find fanfictions about them.
Billy and Mandy themselves are obviously very popular. Fans love to write about their adventures and how they interact with each other. Grim is also a key character that often features prominently. His role as a sort of guide or mediator in their crazy world is always interesting to explore in fanfiction.
One interesting story could be about Billy and Mandy going on a wild adventure in a magical realm that a fanfiction writer created. Maybe they encounter strange creatures and have to use their unique skills to get out of trouble. Another could be a story where they meet their own fan versions of themselves, which leads to a lot of confusion and humorous situations.
There are quite a few 'Mandy x Grim' fanfictions out there. 'The Surprising Connection' is a fanfiction that delves deep into their psychological connection. It shows how Mandy's cold exterior and Grim's dark nature are more similar than they seem at first. 'Mandy and Grim: Partners in Crime' is another interesting one where they get into all sorts of mischief together. And 'The Redemption of Their Bond' which is about how their relationship evolves over time and they both grow as characters.