Said it before, and I'll say it again:
If it makes sense in the lore to allow DH to be a new class, it WILL happen, and frankly, it's more likely to happen than most other ideas (ie tinker). The EXACT same arguments were being made about death knights before they were released: "ALL death knights are evil without exception, it CAN'T happen", "half their abilities are already on other classes in one way or another"
People need to realize, the abilities each class 'has' are based on WCIII, where a unit had 4 abilities max. FOUR. In WoW, EVERY class has many times that amount. Do you remember paladins in WCIII having avenging wrath? Me either, but it's an (if not THE) iconic paladin ability.
Another interesting tidbit is that warlocks and death knights are SO CLOSELY related, that Gul'dan (arguably the MOST notable warlock, even standing in for the role in hearthstone) is listed as a DEATH KNIGHT in warcraft 2 (
http://www.wowwiki.com/Warcraft_II_units), and don't whine about the source being wowwiki, his abilities in WCII were specifically those given to death knights.
In order to differentiate DK's for Warlocks, Blizzard changed their entire Fu relating to death knights. Death Knights created by ner'zhul/arthas were more combat focused, whereas the death knights of old were more casting focused. Just take a look at their abilities:
http://www.wowwiki.com/Warcraft_II_abilities.
As a matter of fact, almost HALF their abilities are keystone abilities of other classes in Warcraft lore.
-Unholy Armor is essentially synonymous with the warlock spells fel armor and plays like a dark version of a bubble.
-Haste, yes DK's had the ability to magically buff their allies with haste. This is a targeted self/ally buff with a punishingly limited duration, so it is different than the passive self haste buff DK's enjoy in WoW.
-Whirlwind, yep, the druid spell cyclone. To the letter, identical.
Did Blizzard edit this fact out with the changes in WCIII? No. They explained it away saying that DK's were varied in style and the 'newer generation' favored direct combat and disease over spells. This is evident when you realize that the ghost of an old Death Knight haunts SMV and has the iconic WCII abilities (
http://www.wowhead.com/npc=21797#abilities)
What's the point I'm making? If Blizzard thinks DH is a good choice for the lore setting, it wouldn't be difficult in the least to explain that while they harness similar magics to that of the warlock, they feel in essence, more like a rogue.