blood elves, goblins, death knights, brewmasters, pandaren. all some of the most asked for things to be added to wow since vanilla in fact pandaren and demon hunter have been THE MOST asked for things which means demon hunter is the most likely class out of all of the possible classes.
the 2nd moast asked for class is probably something like a dragonsworn or maybe a battle mage.
plus demon hunters have tons of lore to be worked around and theyve been hinted at in multiple quests and by multiple npcs in both cata and mists. for example theres a pandaren in the monk zone that says "i wanted to be a demon hunter"