There's been a lot of debate about different possibilities for new class-based content, but I don't think anyone has taken the pulse of the community on which is the most popular.
New class: Speaks for itself. Examples include bards, tinkers, necromancers, etc.
Class skins: Change ability animations, icons, and names, but keep the underlying mechanics. This could be either a full class reskin or individual specs. Examples would be an undeath reskin for warlocks, including undead pets, Void Knight shadow skin for paladins, or a holy skin for shadow priests.
4th specs: Add another spec to each class, including druids and demon hunters. This would allow some concepts that might not have enough meat on the bone to become a new class to be added as a new spec instead. Examples are things like a fire spec for death knights, an earth spec for shamans, or a blademaster spec for rogues.
New combos: This would open up new race/class combinations where they make sense, or even possibly removing the restrictions entirely. Examples would be a Night Elf paladin, Pandaren druid, or Tauren mage.
Other: Anything else you can come up with. Describe your idea in the thread. Examples would be things like dual classing, a jobs system, or spec overhauls (e.g. survival going back to ranged).
None: If you don't like any of these ideas or just think we have enough already.