Originally Posted by
Katchii
Blizzard has stated before that the player character is, lore wise, coming from a specific branch/ faction of whatever race they are from. All player trolls are Darkspear trolls, all humans are Stormwind humans, all Worgen are from Gilneas, all Goblins are from Kezan, all Demon hunters are Demon Hunters trained by Illidan before the fall of the Black Temple, all Death Knights are hero's that fell during the campaign of the scourge, etc...
Just because you see a Pandaren or whatever [insert race here] out in the world does not mean it comes from the area the player character is from.
That said, that doesn't mean that at some point in the future Blizzard couldn't revise it to now allow races to be new classes by updating the training part. As others have stated, the Demon Hunter class hall still exists and has plenty of people there capable of training new Demon Hunters. Same with the Death Knight class hall, with the rise of the new four horseman, a new breed of Death Knights could be created and trained.
However, until the update occurs we're stuck with the current paradigm. I also don't see a point in new Demon Hunters since the Legion threat is essentially thwarted. Death Knights though, at this point I don't see any reason why all races can't be Death Knights given that in the past expansion we saw all races fighting the Legion and therefore have dead heroes of every race able to be raised into Death Knighthood.