One idea could be that they discover they are going to be parents unexpectedly and have to deal with the reactions of their friends and family. It could be a comical situation as they are both rather unconventional characters. For example, Grim might be shocked and initially try to find a way to reverse it, but then slowly warm up to the idea of being an 'uncle' of sorts.
Perhaps there's an alternate universe where Billy and Mandy are older and in a relationship, and the pregnancy is a natural part of their life progression. But they face challenges because their personalities are still so unique. Billy might be overly excited in a very Billy - like way, while Mandy has to be the more practical one, trying to figure out how to raise a child in Endsville, which is full of strange and dangerous things.
A possible plot for Billy and Mandy crossover fanfiction is that they end up in a world of magic school. Billy is completely clueless about magic at first and keeps causing chaos in the classes. Mandy, on the other hand, is intrigued by the different types of magic and tries to learn as much as she can. They get involved in a competition between different houses in the school, and their friendship is tested as they face various challenges and temptations during the competition.
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.
Perhaps in a fanfiction where there's an alternate universe. In this universe, Billy and Mandy are normal kids without all the spooky stuff around them. They participate in a school play where they have to kiss as part of the script, and this kiss makes them see each other in a different light, starting a romantic sub - plot in this otherwise normal world for them.
Perhaps they could be in a modern - day high school setting where Mandy is the cool, mysterious girl and Billy is the nerdy but kind - hearted boy. They get paired for a school project and at first they clash because of their different personalities. But as they spend more time together, they find common interests like a love for old horror movies and start to develop feelings for each other.
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.
One idea could be to explore what would happen if Billy's stupidity was magically transferred to Mandy for a day. It would completely disrupt their usual dynamic. Mandy is used to being in control and smart, and suddenly having Billy's lack of understanding would put her in all kinds of comical and difficult situations.
Well, in Naruto Billy and Mandy fanfiction, a cool idea might be the sharing of abilities. Imagine Naruto teaching Billy some basic ninja moves like the Shadow Clone Jutsu. Billy could then use it in his own wacky adventures in Endsville. Or perhaps Mandy getting her hands on some chakra and using it in her devious plans. There could also be a plot where Naruto is on a mission in Endsville and has to team up with Billy and Mandy to defeat a common enemy, like a super - powerful demon that's causing chaos in both their worlds.
One interesting story could be where they have to deal with the chaos of living together in the underworld. Billy's clumsy ways might constantly create problems, while Mandy's cool and collected nature tries to fix things.
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 might be a fan fiction where Billy and Mandy swap personalities. Billy becomes all serious and Mandy turns into a goofy, carefree character. This could lead to a lot of humorous situations as they try to adjust to their new selves and still deal with the underworld tasks they often face.