A while back, I speculated about the possibility of introducing sub-zones inside dungeons. I#ve just implemented the first iteration of these this evening. These differ from vaults to a degree in that they don't follow any specific pattern, nor are there any greater rewards. Effectively, they're a little part of another dungeon branch inserted into the current dungeon. Monsters will be scaled to be slightly out-of-depth (but not too much), and I'm considering allowing traversal into a different dungeon branch by means of portals contained within these zones (though those aren't implemented yet).
Here's an example of a section of the Plane of Water intruding into the Plane of Fire. Some denizens that normally frequent the Water Plane have came through as well.
The framework is in place for these zones to be dynamic - perhaps they could shrink and expand over time depending on the actions of the player character.