They look cool. They have awesome looking weapons and attitudes. They're among the most popular classes in WoW. However, you're probably never going to see them appear in the game as a playable class. That's not to say that it's impossible, just HIGHLY unlikely. If Blizzard announces the Demon Hunter class at the next Blizzcon, I'll happily eat crow, and prepare to roll one late next year when the next expansion releases. However, until that unlikely event occurs, here's the 5 reasons you'll more than likely never see a Demon Hunter as a playable class in WoW.
Probably the biggest thing hampering the introduction of a Demon Hunter class is that its primary abilities have been farmed out to other classes. Rogues have Evasion, Priests had Mana Burn, and Warlocks have Immolation Aura, and Metamorphosis. Now keep this in mind, when Blizzard introduced the Monk class, they took the entire Brewmaster ability set and applied it to the new class. The same thing occurred when Blizzard introduced Death Knights in WotLK. This wouldn't be a problem if the Warlock and Rogue abilities weren't exactly like Demon Hunter abilities, but they are. Metamorphosis is exactly how you would picture the WC3 spell in WoW. Demonology locks even transform into Demons the way DHs would if they were in WoW.
I really can't stress how important this point is. There's no way Blizzard is going to introduce the exact same mechanic for 2 classes. Also if they did, what would they give the DH that would be different from the Warlock? The Warlock has every conceivable ability that could exist from Metamorphosis. There's nothing more that can be created or taken from that ability to make it different for the DH. Now Blizzard could make DHs without Meta, but what would be the point? Meta was what made DHs amazing in the first place.
BTW, I personally feel that Blizzard made a colossal misstep by giving Warlocks Metamorphosis and DA. I would have preferred a DH class utilizing those abilities. Now thanks to that move, we're more than likely never going to see DHs in this game. Nice move fellas...
Another issue is how DH gameplay would work over three specs. DHs have a pretty uniform style of play. Its so uniform that going outside of that means that you're not really playing a DH anymore. Death Knights also had this issue. To compensate, Blizzard took abilities and concepts from every undead hero and fused them into the DK class. Blood came largely from the Dreadlord, Frost came from the Lich, and obviously UH came from the Death Knight. It all made sense because the Undead/Scourge all came from the same general source of power, so it was conceivable that the Death Knight could legitimately pull from all of those sources and make a varied style of gameplay.
But what about Demon Hunters? I mean yeah, you have the WC3 hero with its four abilities (which are already used by other classes), but what else can you do with it? The Night Elf heroes and units don't really go well with the Demon Hunter concept, and most of those abilities are taken by other classes. How are you going to take that narrow style of gameplay and split it into three distinct specs without significant overlap? Worse, how are you going to avoid making it feel like a do-over of Rogues?
3. Class Balance
Though not as major as the first two, this is another problem with bringing DHs into WoW. DKs and Monks fit relatively well with the game's structure. DKs took the third spot for plate, and Monks took up the third spot for leather. Monks even gave some competition with Druids for INT Leather. Both classes also came in as hybrids, giving the game new opportunities to tank or heal. Both also came in as melee classes. Thankfully both classes were hybrids so players could spec into either tanking or healing if a raid or group needed something besides DPS.
DHs present an interesting dilemma in that regard; They would be melee, and they would more than likely have to wear leather. Granted, they could wear mail, but that clashes strongly with the class' lore. Currently, Mail is the last armor type that doesn't have three classes going for it. Mail is semi-heavy armor. Demon Hunters don't wear heavy armor. In fact, Demon Hunters barely wear any armor at all. At best they would wear leather or cloth. Anyone willing to believe that we're getting yet another leather wearing, agility-based, melee class? Didn't think so.
4.The Illidan problem
Let's be honest; People want to play Demon Hunters so that they can look like Illidan Stormrage. He's probably the coolest and most iconic character in Warcraft. People want to run around with those wings, blindfolds, and Warglaives. People want those runes over their body, and they want to wreck stuff the way he wrecks stuff. So how would this work in-game? Probably not too well. If you introduce Demon Hunters, you're going to see a surge of Night Elf male characters popping up all over the place. None of them are going to want to use regular swords or staffs, or whatever else DHs could potentially use. They would all want the Blades of Azzinoth on their backs, and anyone without those blades would be considered a noob. Would they want to wear helmets, shoulders, cloaks, and chest gear? Nope. They'd run around shirtless just like Illidan. They'll never want to raid because they couldn't look like Illidan while their raiding. You think Blizzard is going to create an entire item system just for DHs? I mean, they could, but I seriously doubt it.
It'll be an utter and complete disaster.
5.Better class options available
Despite their insane popularity, DHs aren't the best class options for the game. Several classes would be a better fit for modern WoW, and offer more to the playerbase, and overall class balance. Tinkers, Spellbreakers, and Rangers are just a few examples. Obviously though, none of those classes have the same appeal as Demon Hunters do.
Conclusion: Given the reasons above, I think its highly unlikely that we'll see a Demon Hunter class in WoW. Again, I'll happily eat crow if I'm wrong, since Demon Hunters are one of my favorite heroes from WC3. In the end, if you want to play a Demon Hunter in WoW, you probably should roll a Warlock.