My point was that we haven't needed a single new class since Vanilla WoW but if we ARE to get a new class, it can be ANY class regardless of how much of it's abilities might already be in play by already existing classes.
Monks were added to the game with almost NOTHING unique. From stuns, to ccs, to burst, to anti-cc ALL of it has been done in so many different forms and with class homogenization EVERY class has some form of another classes' ability or another. Adding a new class need not be hurt at all by already existing abilities.
All it would take fit demon hunters right in is to make the warlock's metamorphosis more demonic. The warlock itself is as demonic as they get. A demon hunter, in principle, is close to a rogue who's discipline has allowed him/her to control it's beast and transform at will into a demon that fights demons, not controls them.