Apparenty, the hot "new" thing in RPG design this week is called the "Black Door". The idea has been posted publicly, and from a few private sources that I've seen...
Quoting +Michael Prescott...
"A Black Door is a dungeon obstacle (in Ben Robbins' original West Marches, an actual black door sealed by magic) that clearly separates skilled, versatile, resourceful and powerful parties from less experienced ones. Often adventurers will be completely unable to get past until they return later, having levelled up a bunch or brought special tools or hirelings."
It sounded familiar...in fact I discussed the idea back in 2010 when I developed my Vector Theory of RPGs.
If we consider the story to be a path, and decision points to vary the course of that path, a "black door" is a temporary terminus to the path...unless a character has the relevant key...in which case the terminus vanishes and the story path continues.
It's a common technique in a lot of dungeon design to channel a specific story through what would otherwise be an open sandbox environment. It can even be used to describe certain encounter types, a werewolf cannot be beaten unless the characters have silver weapons. Without the silver, the werewolf stops the characters dead in their tracks...but with the silver it becomes a minor inconvenience. Even something as simple as wearing the right clothes so you can attend the ball...sure there may be other ways around the obstacle, but the clearest option is to make sure you have the "black door's key", then pass straight through it.
It's funny when ideas I'd considered years ago suddenly become "cool". Sure, I'll admit that the West Marches goes back even further than my posts regarding the idea, but people theorising about them now...as though no-one had considered them from a theoretical perspective...that ain't new.