Just have a theme that doesn't have either a strong central plot-contrivance and/or character - hence the proposed theme of exploration in the place of a faction war, a world-ending threat, or the rise of an ancient evil. For me at least, the best part of BfA was actually the leveling continent and exploring zones like Nazmir, Drustvar, Stormsong Valley, and Vol'dun. The worst part was the War Campaign itself, and the faction conflict it was hitched to. It was similar with Legion, although I think the central story of that expansion was just of a high caliber than BfA (and admittedly dealt with subjects that were just more interesting to me from the get-go). Personally speaking, I think many players in WoW are a bit bored (or perhaps numb) as concerns spectacle without substance.
That is of course just my $0.02 - YMMV as others are concerned.