Cloth wearing DW melee specialist that makes use of shadow magic.
Doable. You'd need a resource mechanic VERY different from that of a rogue. A combo based system might work well. You'd need to look at gear. Do they use INT Cloth? Do you develop specialist cloth just for them? Do you make them wear leather? Do you go the whole hog and give them Mail? What about the Deconstruct idea floating around? You end up looking very much like a DH but get the druid syndrome....essentially looking the same every patch.
Metamorphosis? Signature ability but warlocks have it. You cannot remove it. So you'd have to either not give it to them or give them a different form. Maybe class quests - at level 20 you get a mission to go out and absorb a demon. The demon you choose dictates your form. Or maybe just give the DH the Warlock form as baseline.
The problem, even at this stage, is that the more you do to make it work, the more you either step on other classes or the more you take it away from a Warcraft style DH. You end up with a WC3 style DH that isn't needed or a new class that isn't a DH.
The best I can see is either a reworking of the Warlock to let them fill the role via that Glyph, or simply integrate a D3 style DH and brush over the differences.