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.
Granny from the series could be a great addition. She could act as a sort of wise advisor to Mandy and Billy, providing them with valuable advice based on her own experiences. Her no - nonsense attitude could also add some humor to the story, especially when she scolds Billy for his silly mistakes or when she encourages Mandy to be more open - hearted.
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.
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.
You can also start by introducing a new character who challenges their relationship. Maybe a new girl has a crush on Billy and Mandy gets jealous. In the heat of an argument to claim Billy as her friend (or more), she kisses him. This would create an interesting starting point for the fanfiction.
One common theme is the clash of different personalities. Billy and Mandy have very distinct personalities compared to the RWBY characters. So, there's often a theme of how they adjust to each other. Another theme is the exploration of new powers. Since they are in a new universe, they might discover new abilities or ways to use their existing ones.
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.