Personally, I'd like to see the pure DPS classes done away with and if not adding a 4th spec, at least tailoring one of the three to be something else.
Rogues could have their Subtlety switched to an avoidance tank spec.
Warlocks could have their Demonology switched to a tank spec with metamorphosis. They could also maybe get a 4th spec of a healing tree using dark magics to heal. We already have a few examples from the shadow priest spell set. Those could easily be expanded on. In addition, warlocks already have siphon life and drain life.
Hunters could have their Beast Master spec switched to a pet tanking spec.
Mages could have their Arcane spec switched to healing tree with time and reverse damage as their focus.
Shaman I think are the biggest argument for a tank spec. Back in Vanilla WoW, Shaman could off tank fairly well. Seems easy to bring back some of those talents and expand slightly on them.
Then, there's no excuse of "I can't find a tank or healer!" By doing that, every class would have the option of at least two roles. There would then be 9 tanks and 8 healer specs in the game to work with.
On a personal side note, I would love to see priests get a holy DPS spec. I'd personally push for a fourth spec in that case, as Disc and Holy are quite different in play style.