Actually they pretty much don't fit into the thematic elements of the lore for the entire history of Warcraft. In fact there are several older posts, wikis, etc where it has been expressly stated this would be terrible in this game.
At this point it is going to be hard to justify any 'established' classes into the game. I think things from scratch, sorta like the Monk will work best going forward.This way there is an explanation for why we haven't seen any running around. I don't really want to have new classes retconned into the WoW lore, even if they already exist in the established universe.(I'm looking at you Troll druids :P )