Return everything to how it was back during Mists with the included new spells from gaining more levels. That means Challenge Mode dungeons come back, Reforging comes back, every kind of vendor comes back, justice, valor, conquest and honor points come back (yes i know honor points never left but they've changed how they are used now), class designs revert back to what they were except for minor changes because of new classes (looking at you warlock/demon hunter), heroic/normal dungeons come back as well as heroic and normal raids (they can leave flex mode in the trash for all I care) and every spell that's been removed since Mists is returned. Do that and watch the people flock back to the game in amounts never seen before.