I'm actually curious if they are going to do this with zones too. Perhaps they will add more level 60 zones with more dangerous mobs as player power progresses?
I'm not even sure which zones they could add at this point. They already kept Quel'thalas out of the game because 1) it's the focus of the current Retail expansion and 2) it was originally TBC content and they probably wanted to emphasize on the Classic aspect of things.
The way they worded it, they said "we aren't quite ready to go to Quel'thalas yet" so it seems to be in their plans, probably once Midnight is fully over. But it was a big part of the Warcraft 1 to 3 story so it's a bit jarring.
Now this is an alternate timeline where the portal to Outland is never reopened by Lord Kazzak, Northrend is never explored again after Arthas returns and kills King Terenas, the Cataclysm never happens, Pandaria never discovered, etc etc... so where do you go? You can't add the Azuremyst/Bloodmyst isles, you can't have any new landmasses or planets or dimensions.
They've been tinkering with "Heroic World Tier" in Legion Remix and in the next Midnight patch (plus the Val/Naigtal sub-zones in current Midnight), so perhaps they'll add a toggable Heroic difficulty to outdoors content so players at max power with raid gear can go to a place and not steamroll everything.
That's the limits of this concept I guess. They said they want to fill in the blanks on the Classic maps but after they added those two new zones, there's not much left right out the door (except Quel'thalas).