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.
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.
Billy could develop by becoming more responsible. He might start thinking about the future more seriously instead of just his usual carefree self. For instance, he could learn to save money for the baby or start looking for a proper place to live. Mandy, on the other hand, might show a softer side. Usually tough, she could become more nurturing and protective, showing that there's more to her than her intimidating exterior.
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.