Interesting idea, but I feel this would take away from the 1-60 experience that was (will be) the heart of Classic. You could be in the middle of a zone at level 30 and need to stop too.
Also, development doesn’t work quite in the way of like working on content from level 1 to 60 in that order. For example there could be a lantern or quest that is being fixed that affects 3 completely separate zones (this is just a random example off the top of my head, I’m not referring to anything specifically!)