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 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.
One common theme is friendship. Their unique dynamic can be explored in many ways, like how they support each other through tough times. Another theme could be adventure. Writers might send them on exciting quests or into mysterious places.
One popular 'billy and mandy fanfic' could be 'The Adventures of Billy and Mandy in the Enchanted Forest'. It takes the characters on a journey full of magic and mystery. Another one might be 'Billy and Mandy's Friendship Test' which explores their unique relationship in different situations.
A possible plot is that the kiss was part of a bet. Billy lost a bet to Grim and as a result had to kiss Mandy. But then, unexpectedly, Mandy didn't mind as much as she thought she would and it led to a series of comical and heart - warming events in their little group.
Sure. You can check out fanfiction.net. There are often some well - written Mandy and Billy fanfictions there. You can search by the characters' names and explore different storylines.
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.