I wouldn't be concerned about variety, all it takes is a little imagination and you can spin concepts out into highly varied environments - just look at Outland, Northrend, and even Khaz Algar, all variations on a theme (alien, cold, underground).
For zones, I can see
Eversong as your classic gorgeous fantasy elf forest, include the revamped Silvermoon City, tons of Void incursions, magical buildings and settlements, etc.
Ghostlands can be your haunted, corrupted, dark forest where there's still a lot of the Scourge/Undead elements, with wasted zones and graveyards, some resurgence along the Dead Scar.
Zul'Aman can be a mountainous evergreen forest zone, plenty of opporunity for high mountain peaks and vertical travel there, forest troll cities, etc. 'Misty forest' like how Worldshaper said.
And I can see a brand new zone being added to the west, the '
Northeron' region that's north of Stratholme and so forth. Turn that into a coastal zone, maybe where the Arathi Empire makes landfall, huge fleet, also massive naga incursion, Undersea elements being called forth by the Void, fortresses being built, fortifying nearby islands, etc.
That gets you your big starting four even before we talk about Quel'Danas, leaving it as a potential patch zone (maybe some starting content there) with a major Void foothold around the Sunwell (you know that thing's gonna be the Voidwell at some point).
I can definitely see the conglomeration of zones you were talking about happening though. If not for Midnight, then in the future for some major Lordaeron update.
I had some thoughts about that myself and took the same idea of merging zones together to make larger, DF-era sized ones.
They go wind up going that way for sure, merging Lordaeron zones together. I'm just really fascinated to see what would happen if you took a much older zone and rebuilt it from scratch with modern aesthetics. Expanding it out, making it closer to what it was truly 'meant to be'.
Totally, that's troll country right there.