For these purposes, we are going with the following definition of Mass CC: "Any non-weapon non-shield non-creature card that has the potential to kill or negatively imbue more than one creature without targeting any single or multiple creatures specifically."
The following cards currently exist in the game under this definition: Pandemonium, Plague, Rain of Fire, Flooding, Thunderstorm, Unstable Gas
This leaves Aether, Darkness, Earth, Gravity, Life, Light, & Time as possibilities for the Card Designer task.
(Thanks to 10 men for his help on this.)