I agree that warlocks and Demon Hunters are different Lore wise and I'm sure if they ever made demon hunter they'd have different gameplay, more melee based. I think the only problem lies in the fact that many of the demon hunter's abilities, at least the ones we know of from warcraft 3, have been used up for different classes. So although I think it is possible to make them a separate class, creating new abilities that wouldn't overlap with other classes would be an issue. again still possible but maybe not enough diversity since much of their abilities have been given to the existing classes