I also can't even conceptually come up with an idea how they could add a new tank spec at this point.
They already had to go deep into unique mechanics to make Brewmasters unique (replacing armor with the Stagger/Purify mechanic), but their active mitigation and general survivability ended up being largely a cross between Druid and DK as it is. The design space for tanks isn't huge to start with since tanks have a very specific set of needs that can't be easily diversified like DPS. I've yet to see anyone, anywhere, come up with a tank spec that would be functionally unique and add something new to the game.
There's also the problem with too many healing specs - we've reached the point where a even a 25 man can't bring 1 of every healing spec to most fights, and to a lesser extent, healing specs have the same problem as tanking ones - there's only so much space for mechanics to make health bars stay full.
I'd suspect that the next class added, if they add another, will be pure DPS - maybe a pure DPS spec that has both ranged and melee specs to mix things up, or as part of a package of class changes that add that "support" role they were talking about.