Why Demon Hunters won't be a thing: There is no thematic, lore or gameplay niche that demon hunters could fill at this point not already covered by some combination of warlocks, paladins or rogues.

Rogues do the whole Demon Hunter stabby-stabby thing. Paladins amongst other things turn evil and fight against the forces of darkness. Warlocks; well... They're warlocks. Demonology warlocks already have half of the Warcraft 3 Demon Hunter abilities.

Another reason I don't think there'll be demon hunters as a class called "Demon Hunters": Basic marketing psychology. You already have a class called "Hunter". In games like WoW, the classes will be pidgeon-holed into clear archetypes to differentiate themselves in players' minds. Obviously Demon Hunters are nothing like Hunters, but they both have the word "Hunter" in their name. As trivial as that is, that's the sort of thing a lead producer will consider to be a problem.

If, big if, Warlords of Draenor is officially a thing, I doubt there'll be playable Demon Hunters. At best they'll be a fourth spec added to Warlocks. Either way, don't expect to play mini-Illidan. It's not gonna happen.