I don't want this to become a Legion circlejerk thread, but honestly, Order Halls and artifact weapons were Legion's best feature.
I've been leveling a bunch of alts lately because I really don't know what else to do in the game, and I love love love the order halls. Doing the class specific quests, collecting all artifacts and their different appearances, it's just so much fun.
It really makes me wonder why Blizzard didn't use them again in any other expansion. I really wouldn't mind if we got another expansion with order halls and artifact weapons.