WoW has survived for almost seven years now, and like Darwin predicted, it did so by adapting. I remember my first experience with this; Blizzard integrated the addon DoubleWide into the standard UI in patch 3.2. As someone who'd only ever played console games, the thought that developers would add changes to the main game after the fact was simply mind-boggling. And I began to wonder: What else could WoW add? What else has it added? To Google I went.
As it turned out, WoW had added a lot. What drew my attention was the most recent addition: death knights. The new kid on the block, they were an oddity due to their unique resources and mechanics, like runes, diseases, Death Grip, and Raise Ally. They were just as resilient as paladins, while not as defensively oriented -- a bit like necromancers, but not really like them. Even the Warcraft RPG's runemaster could be seen in them. There was nothing like the death knight.
However, I constantly jump between games. After a while, I went and found other MMOs to play. MapleStory, Guild Wars, DDO -- the one true constant was the class selection. Warrior, wizard, rogue, archer ... WoW had a version of every class, and every game had a version of WoW's classes. And then I remembered death knights and realized I was mistaken.
There are many classes that aren't represented in WoW's ranks. The one that springs most readily to my mind is the mesmer, a class from Guild Wars that specializes in spell interrupting, delaying, and reflecting, punishing their opponents for action or inaction, and directly manipulating their opponents' resources. Or maybe the fan-created chronomancer is more to your taste? Or maybe the bard?
If you could pick any class, which one would you want to bring to WoW, and how would it play?