Dungeon hubs, like Hellfire, the place in Zangarmarsh, Tempest Keep, Auchindoun, Ulduar, etc.
I really liked those and think they should come back. They created a more interesting location than having each one spread out with one or two in each zone but spread way far away from each other (like how Mists and Cata was).
There's also something to be said for how Coldarra worked, at low levels you take out one part of the complex, then when you're higher you deal with bigger threats, and then at the end it culminates in a raid.