The Death Knight in Wrath of the Lich King was the first class introduced to WoW, and it's inclusion in the game has had mixed reviews. Most folks agree that the class provided a great new element to play in the game, however its balance in raids and PvP is something that is constantly under the microscope. Ghostcrawler has mentioned numerous times that the Death Knight has taken up more time to balance in Wrath than any other class. This is problematic for numerous reasons, but has been able to be handled nicely by the designers and programmers (I'm sure there'll be some who disagree, however show some restraint in your disagreement).
So now for the hypothetical part -- if Blizzard were to introduce a new class in Cataclysm, I would suspect that it would be the healing counterpart to the Death Knight. Mechanics of the class would probably operate similar to the Death Knight's rune system: each small heal or heal over time would be castable based on an available rune, which would in turn generate runic power for larger heals. The class would rely less on spam type heals (such as the Paladin, to some extent), and more on preventative heals (such as the Druid, to some extent).