I've been thinking about a level squish for a while, and it could be done with scaling. Make 60 the max level again.
EK & Kalimdor are revamped and go from 1-60 again with the addition of the Cataclysm zones. Outland, Northrend, Deepholm, Pandaria, Draenor, and Broken Isles become 'Heritage Content' that can be done at 60 and optionally, and is not required whatsoever but brings a few additional bonuses and benefits like achievements and mounts if you complete them.