My top shelf pick would be a world revamp that focuses on tying up loose ends in EK and Kalimdor, possibly just one if both are too ambitious.
Forsaken loyalists and Scarlet Crusaders in Northern EK, Dark Iron radicals with Vanilla Ragnaros/Blackrock mountain nostalgia in the Dwarven zones, and various odd ends like Fel remnants or void fanatics further south.
A more likely plotline would be Light/Void and/or Infinite Dragonflight on the Dragon Isles. Light and Void invades, we need something hidden on the Dragon Isles so we get Dragon funtimes as the levelling story along with dungeons and some raid odd-ends, but with Light and Void as endgame stuff. Alternatively much of the same, except with the Infinite Dragonflight and time-travel stories for flavor.