Ever since Wrath of the Lich King (because Illidan wasn't much of a real threat let's face it), every expansion we've faced some existential threat to Azeroth. I think people are exhausted of 'The next big bad', and we appreciated the mood of Pandaria (Until it shifted into "The world is in danger from Garrosh!"). You can have meaningful raids like Throne of Thunder or Heart of Fear without every villain we face the end of the world.
Which leads me into my point. I think Azeroth needs a Cataclysm type update. Nothing major like revamping quest chains, but introducing scaling levels to the old world, introducing world quests. The tech is already there as we say it with Legion invasions. The design space put towards making new zones could be put towards reinvigorating old zones. How cool would it be if we saw Gadgetzan updated in the expansion after Legion to be like Hearthstone's reinterpretation of it? The entire zone of Tanaris as a giant city to quest in as a updated leveling zone.
Overall I think the world needs a reimagining to introduce scaling content and boosting the exp gains overall to make it so players can just enjoy the storylines they want to follow, and level in zones that they like. Because we're quickly reaching the point where buying level boosts aren't just an option, but mandatory for new players.