+3 on the possibility of a ki-rin threat room. Once had one in the level with the shortcut to Darkforge. But I'm guessing they're as rare as the monsters themselves.

DH provides some interesting tension and threat rooms. I've had molochs (which provided nice corpses and a few suits of hulking armor after a Magic Missile marathon) and titans (they don't see in darkness, but eternium quarrels HURT) of both kinds, as well as chaos knights (lots of eternium gear ).

Mixed vaults also tend to spawn monsters of very high DL compared to the area they're in. For example, a werewolf lord in PC:6.