I personally would love to have demon hunters. And I would be very disinterested in something like a time walker or tinker.
We already had monks, they were called discipline priests.
We already had death knights, they were called warriors speaking purely from a "what can the class do" point of view.
We haven't NEEDED a single class since the first 9 because the talent specializations covered EVERY "hero class" archetype.
If Blizzard wants to make something work, they'll make it work. Discpline priests could have turned into melee healers if Blizzard so wanted.
Warlocks can very easily be converted to full time demon hunters. Hell, hunters could have become demon hunters if survival had remained a melee spec. Mages can be converted to archmages. Balance spec druids can turn into keepers of the grove. Arms warriors can turn into Blademasters. Protection warriors can turn into Chieftains etc. etc.
You know the one thing no one has mentioned yet though and something that would and very much could come to be: priestess of the moon. Mail wearer healer that can finally make ranged weapons no longer be one class exclusive.