Personally I'd be happy if they left the original leveling questing in place and did the following:
- You level up through each zone exactly as you would if they were current content but obviously at a much, much faster pace.
- Allowing heroic dungeons for older content to be viable experience wise even when the next expansion's normal dungeons have unlocked. Currently when you unlock Wrath normal dungeons you've also unlocked the heroic dungeons for TBC. Which you skip over for the normal dungeons.
- Leveling through old content would be like travelling through time to see it all the recent history happen, perhaps they could place some interaction with the Bronze dragonflight to make this idea official. With maybe a bread crumb questline throughout the older zones to support the idea that a Bronze Dragonflight NPC is leading you throuh the history of Azeroth. Would be great for noobies too. It'd be a good explanation for why a Pandaren is in Outland and so on.
- Then, when you reach max level the older zones phase into a recent and up to date version of themselves which Blizzard can update every so often to reflect the world's current state. The phasing can be turned off and on (much like Theramore) to allow you to go back and quest during those older times if needed.
This, in my opinion, is how they should have done it and where they should go with making a more persistent and living world.