Filler is an ugly term, but that's kind of what it is. Legion was huge in terms of story and scale, but people forget that the two expansion before it were WoD and Mists, both could be considered filler (One could be considered landfill). Obviously I know nothing and it's all speculation, but quote me on it in a year or so... I think that the last tier of this expansion will be us against an Old God, setting the stage for a huge Old God expansion.