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.
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.
One idea could be that the Reaper loses his scythe and Billy and Mandy have to help him find it in the Underworld. They encounter various strange creatures along the way.
Once upon a time, Billy and Mandy found themselves in a rather strange situation. Billy, with his goofy charm, started to see Mandy in a new light. One day, while on an adventure in the Underworld, Billy accidentally saved Mandy from a spooky monster. Mandy, who was usually tough as nails, blushed a little. From that moment on, Billy couldn't stop thinking about Mandy, and Mandy began to tolerate Billy's presence more than usual. As time passed, they started to share little secrets and jokes, and a tender love began to bloom between them.
Well, without more context, it's hard to say exactly. It could involve Billy's goofy nature and Mandy's more serious and often dark - humored self. They might have a love story that develops in the midst of their spooky and wacky world. Maybe they first bond over a common enemy or a shared goal, and that gradually turns into love as they understand each other better.
One common theme is friendship. Despite their differences, Billy and Mandy's friendship is always a central part of the fanfics. Another theme is adventure. They often go on various adventures together in the fan stories.