To be honest, they don't need to fit any expansion theme at all.
Worgen and Goblins had almost no footprint in Cataclysm outside of their starting zone lore. What did they really contribute to fighting Deathwing? Monks had very little to do with Garrosh's insane bid for power, and they didn't contribute any more than any other class when fighting the Sha or Mogu. Tinkers are just going to be another playable class if they get added. Their lore is most likely going to be tied to Kazan/Undermine, which is conveniently situated in the South Seas.