Interesting thoughts. I'm having trouble finding where the difference would be between this and just adding in more specs for characters. Like these to me look like four new Priest specs sharing different sorts of spells & talents. If we were to assume that Blizz did only half of these per class, that's still 24 new specs to balance on top of the 36 they are already balancing. Compared to the 2-4 they would need to balance for a new class, that seems like a lot.
Part of where Covenants work that I feel this falls short is that the abilities are mostly shared by all classes. My Necro Monk has Bonedust Brew which is a special ability no other class has, but otherwise her spells are very much the same that a Necro Warrior would have. Different specialties, different choices, but still the same baseline abilities. This I feel would get unwieldy really fast.
That being said, Witchdoctors sound super fun to play, so I'm not totally opposed.