One crucial point behind the popularity of WoW, often forgotten, is, in my oppinion, the way the game allows us to live our Warcraft fantasy in an expansive and breathing world.
WoW provided a away for the legion of Warcraft RTS games fans a way of playing as one those units they had controlled in the previous games of the franchise. More importantly, it gave them a way of playing as one of those Heroic units they had controlled in the previous games.
Your fantasy of being a Turalyon or an Uther? Fulfilled through the Paladin class. A Lothar or a Doomhammer? The Warrior. Guldan? Malfurion? Thrall? Rexxar? Vol'jin? Jaina or Khadgar? Tyrande? Or even one unit not affiliated with any hero such as the Priest, the Shaman, or the Witch doctor.
All of those, you could play the equivalent of them in game. And that was simply amazing. Even if the corresponding class might not be a full copy of those units. The feeling was there.
Of course, not all of our beloved heroes were available. Arthas? Had to wait till WOTLK. Chen? Till MOP. And Illidan? Still waiting.
This is why, even though I recognize that there's very little room for it, that I believe that an incarnation of the Demon Hunter must be made available in game. The possibility of playing as an Illidan is simply essential to the fulfillment of WOW's promise.
You might say the Warlock already fulfills it. It does not. Give that class a melee spec and unlock it for Night Elves and I might accept that. But at the momment? It most certanly does not.
You might also say that wanting to play as an Illidan is simply being a fanboy, and that's not a good argument to be used when it comes to creating a class.
NONSENSE, I say! That's the whole point of this game! Allowing us, Warcraft fanboys, to live out our Warcraft fantasies!
I trust Blizzard to find a way to implement such a class in an interesting way. After what they did with the Monk class and Pandaria, I know they would do a great job.
It might not even be called Demon Hunter, the same way that the Monk isn't called Brewmaster, but I still hope that the promise will, someday, be fulfilled.