Warlock Demonology is the Demon Hunter.
I think Blizzard will continue to make it even more obvious through gameplay in the future.
0.5. Demonic Slash gain from Demon Hunting.
Turning you into more melee focused as Demonic Slash has (Short Range), outside of Demon Form.
More like Demon Hunter. Hence the name, Demon Hunting.
1. Glyph of Demon Hunting
You imbue yourself with demonic energies. Permanent demonic wings outside demon form.
2. Metamorphosis
Temporarily transform into a demon + you look somewhat similar to Illidan the Demon Hunter.
& this is Illidan transformed into a demon.
3. Warlock have the Demon Hunter "tier of Illidan"
The horns of Illidan and markings on body.
4. The Demonology spec icon has Illidan on it.
Illidan, as the Demon Hunter.
5. Demonic Fury is the resource system for Demonology, Warlock.
Same thing most likely Illidan would use, of course!
6. Green Fire just like Illidan shows off at Black Temple..
7. Immolation, same spell that Demon Hunter uses in WarCraft III.
8. Demon Hunters in-game uses ability like Shadowfury that Warlock already has.
You can find Shadowfury on the talent tree of the Warlock, yet another ability used by Warlock that is used by Demon Hunter.
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Look at the existing Demon Hunters abilities in-game.
Theras
Metamorphosis (Warlock-Demonologys ability)
Netharel
Metamorphosis (Warlock-Demonologys ability)
Evasion (Rogues ability)
Alandien
Metamorphosis (Warlock-Demonologys ability)
Shadowfury (Warlocks ability)
Truth be told,
Demon Hunter outside of Metamorphosis act like Rogues.
Demon Hunter in Metamorphosis act like Demonology Warlocks when they are in Metamorphosis.
Whole idea of a Demon Hunter, their known super ability, is what they do in Metamorphosis, they become ranged damage dealers, unleashing demonic fury, the way Warlock does with Demonic Leap, Metamorphosis: Touch of Chaos, Carrion Swarm, Metamorphosis, Immolation Aura, Metamorphosis: Chaos Wave! with green fire that also Warlock can unlock.
Uniqueness of Demon Hunter known thru Illidan and others as you see above, is already used strongly and more by the years thru Demonology.
And if you want to ignore that, you are a Rogue.
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Illidan Stormrage has demonic wings outside of his demon form.
Warlock-Demonology has demonic wings outside of their demon form, with Glyph of Demon Hunting.
Illidan Stormrage has demon form in his demon form.
Warlock-Demonology has demon form in their demon form.
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I'm not trying to be funny, if you're smart enough, you'll see the same thing I see.
Blizzard will probably just continue this way with Demonology, for it is the "unofficial" Demon Hunter class.
Therefore, there will be no new class called Demon Hunter, because it already exist.
So if you are one of those who really want to be a Demon Hunter, play it now - it's called Warlock \ Demonology.