Well, you could consider the "escape" approach (look up "Escape from the Lich King" in the dungeon Halls of Reflection), or urgency in the form of an additional boss fight or so that can yield a unique reward. Say, as you enter a Garlean military compound-type dungeon you run into an informant for the party leader's grand company that tells the group that there's a bunch of prisoners that are going to be executed in X minutes, forcing your group to hurry and rescue them to get the extra reward. Maybe to avert the execution you have to fight the executioner.

The "one-shot if you don't get away fast enough" works better as a boss encounter of sorts rather than a whole dungeon, though.