The cosmic chart that was included in the preview of World of Warcraft Chronicle Vol. 1 is an absolute treasure trove for those of us who like to break out the tin-foil hats. I've written about the possibility that the Lich King might have existed long before Kil'jaeden caught Ner'zhul, and I think there's a lot of speculation to go around about the nature of demons and the Burning Legion and why the Shadow has two primary representatives while the other Great Powers have only one.
But much of that is concerned with the outermost layers of the cosmos. Go in a step and you'll find the Elements. There are the four classic alchemical elements, Earth, Air, Fire, and Water, which are pretty much what we've spent much of WoW dealing with.
We've often heard that Shamans also work with a "Fifth Element," and no, it's not love. It's "Spirit," which is the sort of ineffable force of life. Shaman healing spells have become more water-focused over the years, but there was a time when they were more about just kind of green life energy. Spirit is the kind of animating essence that makes us more than just some mixture of physical elements, and it's what the Shaman calls upon to restore life through spells like Reincarnation, Ancestral Spirit, or Cleanse Spirit.
Shamanism is a very common practice across all of Azeroth (and Draenor of course.) The Pandaren have Shamans, but Pandaren also developed a special interest in this fifth element. The Pandaren word for that element is Chi, and it is the basis around which their Monk techniques are formed. Monks use some shamanistic or shaman-like techniques, using the power of wind or mist (you could argue that Brews might have the element of water in them,) but they channel those powers through themselves rather than calling on the elemental spirits themselves. In a sense, you could almost argue that Shamans and Monks have a similar relationship to Priests and Paladins or Warlocks and Demon Hunters, respectively.
We know that there is another side to Shamanism, though. The version of Shamanism we're familiar with is one that seeks balance and harmony, and asks the Elements to grant their power. But Dark Shamanism is another version of the practice, and rather than asking, the Dark Shaman takes the power by force. The Dark Shaman were probably never as prominent as during Garrosh's reign over the Horde, and made up his primary magical forces while he was trying to create the "True Horde." We saw the result of his Dark Shamans work as the landscape in the Northern Barrens was rendered even more damaged and polluted. Dark Shamanism is seriously nasty for the land, and seems to poison and ruin it.
With the cosmic chart, we see that there are six items within the "elements" ring. There's Fire, Earth, Water, Air, and Spirit, but there's also Decay.
I don't think it would be too much of a leap to guess that Dark Shamanism works by swapping out the use of Spirit for Decay. They still use the four physical elements, but they now sow death and, well, decay.
So that might be the simple explanation for Dark Shamanism. But what about Monks?
Monks, after all, use Chi (Spirit) to fuel their techniques. Would it be possible, then, for an order of Monks to arise who use Decay instead?
Admittedly, if there's one class that seems to be in the realm of decay, it's got to be Death Knights, and their necromancy might reach down into the sphere of Decay to do what it does. But I think it's an avenue that's open for Blizzard if they want to create an order of Dark Monks.
Unless that's just Rogues?