Some form of entertainment, written as if it was by aliens, for aliens from the same culture. So, for instance, if they were writing their version of Stargate SG-1, all of the references to the Simpsons, mythology, etc. would be explained to the exact same degree that they are in canon!SG-1. And maybe idioms would be different, too, depending on how far you want to go with it.

Vacation's Over

A long time before the actual story, a group of heroes (I'm thinking the Scoobies, but it works with any heroes), after years and years of saving the world and becoming immortal, are rewarded by the Powers: a guarantee that the world and themselves will be kept safe and they will be basically blessed, and that they can just go off and enjoy life. It doesn't come too soon; the heroes are all weary and sick of fighting, and ready for an extended vacation.

Fast forward to "the present". A bean counter in the Powers notices they're expending a lot of power on people who, as far as he can tell, do nothing, so he or they cut that power off.

The heroes notice. By this point, they're rested and willing to rejoin the fight, and getting kind of bored now that the power which has been keeping them happy has been cut off. So they go back to the hero business. Experienced and rested, they're better at it than they were the first time around, and bounce around solving problems and causing chaos.

The time the Key can be used is actually calculated based on the year of a planet other than Earth, possibly one with a very short year. Optionally Glory isn't able to open a portal. Either way, sometime after Sunnydale turns into a crater, Dawn cuts herself (or is cut by somebody else) and finds herself in another world.

The roles of the characters of X fandom are swapped as much as possible. Obviously some roles are situational or personality-based rather than random enough to swap (SG-1's roles are (military) hard scientist, soft scientist, military, and alien warrior. Some could be swapped easily; others would take major rewriting of backstory and even names- Sam Carter is the former first prime of Apophis...but then what about Jacob? Why do they have American names? Etc.), but for instance I could see it going well with Buffy: Willow the Vampire Slayer! Xander the witch! Buffy the token normal person! (Jenny the Watcher! Giles the Gypsy!)