From this point its a case of finding how to fit a class design into the needs, and making it work with the lore. Just because the popularity of Demon Hunters both WC3 and D3, is why it seems to be the path of least resistance. Do you think they can get 1.8 million players back with a Tinker class? Since Gnomes and Goblins are the least popular races in the game. Or will they go with an Iconic class used by a popular race. ie Night Elves and Blood Elves.
You seem to forget Blizzard is making a product which needs to be sold to gamers.
Of all the suggestions only Demon Hunters and Necromancer would be popular enough to revitalize the game.