As long as they planned it out correctly a "filler" expansion could be great. Island hopping, discovering tribes and cultures tied to specific islands, lush jungle islands, desert islands, icey islands, etc. With as the endgoal breaking trough magic mumbo jumbo preventing us from entering Kul'Tiras with the McGuffins we collected on said islands. As the patches go on the theme starts to shift from "Yay adventures and islands!" to "Why are there tentacles with eyes on them growing out of the ground?: