I really like your approach on this. I understand that it would take a ton of resources though... but eh, a man can hope right?
I would be more than fine if we get those meta-zones in patches to give Blizzard more time to work on them. In the meantime though, I think they should not be available at all, instead of having old zones co-existing with new zones which destroys the immersion. Just put the whole of Cataclysm Azeroth in the Caverns of Time and rebuild from scratch. And if only a third of Azeroth is available at first: then good, it will create a sense of mystery again to a world we now know by heart. Rediscovering Khaz Modan or Stranglethorn Jungle in an updated, HD experience is much more exciting to me than any "new zone" would ever be, and the game really needs it. I remember Gilneas when it was just a closed giant gate south of Silverpine Forest, and man, that was something: because it added an element of mystery ot the game and made us wonder: what's going on there?
A timeskip would also help with explaining why some zones are unavailable at launch. It could bring some potential new plots and storylines such as the light inquisition in Lordaeron, political struggle in Khaz Modan leading to the whole kingdom shutting its borders until patch 10.X, pirate and Gurubashi raids in Stranglethorn, Qiraji and Titan-affiliated forces in Southern Kalimdor, etc.
There is so much to do with the current state of the world, and so many loose ends and races which have never really been used to their fullest potential: ogres, centaurs, high elves & void elves, furbolgs, and so many others.
BfA has turned the world into even more of a mess, but I think all things considered it's a good thing as it has allowed the world to move forward in terms of storylines and as it planted the seeds for future developments and a potential world revamp. Now I can only hope Blizzard will one day address the elephant in the room... the game has aged terribly and a fresh start would be more than welcome.