But WoD -> Legion is really the only transition we need to be concerned with. They said sometime after MoP (or was it during WoD?) that from now on they would build expansions where stories connect to form a bigger narrative. So MoP had the breadcrumb for WoD (racist Garrosh), but WoD and Legion are really part 1 and 2 of the greater Burning Legion narrative.
Similarly, the next expansion will likely begin the Shadows of the Void (or w/e) narrative with perhaps Sylvanas, Argus, or Jaina as a breadcrumb.