I couldn't think of term to apply here, so using the examples of what we've had already seems appropriate.
Each expansion since MoP we've had a character that is a central, driving force in a narrative sense. The character that anchors us to the story, leads us in the proper direction, and sometimes placed firmly in the middle of the crisis so that we are aware of what is going on. Some people call them the "face of the expansion" and I've seen other people to refer to them as the "expansion narrator" which I can understand.
So we've had Lorewalker Cho for MoP, Khadgar/Durotan + Draka/Yrel for WoD, Khadgar (then Illidan) for Legion, Magni for BfA, and now Bolvar for SL.
If next expansion we are to have another narrator-NPC-character... who do you think it would be? Or who would you want it to be?