DH will probably never get a third spec. When DH was revealed to only have two specs, Blizz was very open with why. They initially set out to give DH 3 specs, but having 2 DPS specs felt forced and just meant they were splitting the cool ideas they had over two specs. They didn't have two meaningfully different melee profiles and adding in a range spec felt forced because they felt DH was inherently a melee spec. It also would have meant that Vengeance wouldn't have gotten things like Throw Glaive and Eye Beam, because those would have gone to the Range spec. So, in the end, they decided to do just one melee spec because that is what felt right.
They even elaborated that they thought there were other classes where two specs would have made more sense than 3, but the original philosophy in WoW was that all classes would have 3 specs. I believe Rogue was specifically name dropped here, where 3 specs feels forced and that the differences in play style could likely be achieved just as well through talents as through separate specs.