You should have started this exercise in October. Anyway, here's mine:
The current iteration of God and Satan (or whatever fictional supernatural character you choose) BOTH suddenly go AWOL and throw the entire system of heaven and hell into a quandary. Turns out they are both vacationing on Earth on some remote island, spending their days fishing and drinking beer.
Romantic twist: One is male, the other female and it turns out they have eloped and plan to spend the rest of their lives together.
Thriller twist: They get fascinated by the concept of zombies and start creating a zombie army to serve their needs. The existence of Earth is threatened.
Sci-Fi twist. Heaven and Hell are singularities at the two ends of the universe. Thanks to the disappearance of the two, the singularities are unstable and about to collapse into one unholy (pun unintended) mess.
S&S twist: God and Satan are names of Dragons who copulate and the eggs they lay create new universes. Them going AWOL is a traditional indicator of the end of one universe and the beginning of the next.
ASOIAF twist: Each religion gets to be God and Satan for a specific period of time. The current iterations (the ones that have gone AWOL) of God and Satan are killed and the blame is laid squarely on the representatives of one of the religions. Other contenders to the thrones of Heaven and Hell emerge.
Take your pick? :)