They skipped both a new class and a new race for the first time with the last expansion (another aspect of decreasing quality / content). It's hard to say if they pick it up again or not since they've been whining how hard it is to keep up with the current classes (which it probably is with their gutted support team).
Deathknight was supposed to be a hero class which meant not just starting at an elevated level and using a different ability system (i.e. not mana, rage, energy or focus), but it was also supposed to be something that required more skill to play. Blizz abandoned the "more skill to play" aspect during beta for Wrath. What a deathknight was supposed to be varied throughout that expansion. Blizz's intention at that time was to introduce a new hero class with each expansion.
Monk was one of those ideas (along with necromancer and runemaster...bard was an April Fool's Day joke) that got turned into a regular class.