I think you skipped right past the part of my comment where I talk about the "fantasy" a spec needs to fulfill. The situation with Demon hunters was different because it was a demon powered melee class vs a demon powered spell casting class. There was room there to fulfill another fantasy with two specs, huge brute demon monster and skilled warglaive wielding fel ninja. They even said they struggled making more specs and kept it to two because they couldn't think of a good third.
Of the possible powers that exist to differentiate the Dark Ranger (shadow, void, dark), there is only one fantasy fulfilled: A bow wielding stalker that uses dark magic. Unless you can come up with a meaningful difference between the specs outside of the shade of their purple, you are going to struggle making a successful spec.
You can force in a healing or tanking spec but how many people think: "Yeah dark bow wielding healer! That's the spec that fulfills my fantasy!"