Some concepts could involve time - travel. Naruto could be sent back in time to when Billy and Mandy were younger and change the course of events in Endsville. This could lead to a whole new timeline where things are very different. Another concept might be a battle of wits between Naruto's strategic mind and Mandy's always - plotting nature. And of course, there could be a story about Naruto trying to understand and adapt to the very strange rules and creatures in Billy and Mandy's world, while Billy and Mandy are equally fascinated by Naruto's world of ninjas and jutsus.
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.
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.
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 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 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.
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.
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.
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.
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.
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.