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.
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.
Well, obviously Billy and Mandy are the main characters. Billy is often the goofy and naive one, while Mandy is more cynical and in control.
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.
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.
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.
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.
Most Billy x Mandy fanfic often has a spooky or supernatural element, just like the original show. It usually centers around the odd couple - Billy, the dim - witted but good - hearted boy, and Mandy, the cynical and somewhat menacing girl.
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.
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.