Kind of like the Knights of the Ebon blade, but made of demons.
A faction of Eredars, Dreadlords, Pit Lords,etc defecting the Legion (for X reason) and making a pact with the Azerothians to defeat it, not necessarily because they are "good demons" but because they share the same goal. A enemy of my enemy is my friend type of deal.
That'd create some hilarious interactions and add in not previously explored drama in the storyline. Do you imagine, watching Draenei and Dreadlords cooperating? Non-Legion Demons that just want to be left alone and that don't care about destroying worlds? Eredars seeking redemption?
I think WoW portrays demons as being too one-dimensional. For example I wanna know what kind of society Dreadlords live in. We already know they are damn good craftsmen (they created the Lich King's armor and Frostmourne), but what about architecture? What kind of cities do they live in? What do they do in their free time? Hell, are there female Dreadlords? Do they procreate and have baby Dreadlords or just spawn in the Twisting Nether all grown-up.