Blizzard has the techniques to make zones more efficiently than they were a decade ago. For Dragonflight,
Blizzard's environment artists have created a new technique where rather than hand sculpting by hand the entirety of a continent's geometry in WoWedit, they instead have created modular pieces of terrain that they can quickly copy paste together.
I don't think biomes/zone aesthetics would need to be consolidated as @
George Lucas posited. Look at the Waking Shores, a zone which has two biomes/tilesets. If you overlay the Waking Shores onto Kalimdor, it covers the same area as Ashenvale and Winterspring, which are also two biomes/tilesets.
The density of different biomes or zone aesthetics has not changed, the only difference being the quantity of zones/size of the continents that Blizzard releases with every expansion every two years. Kalimdor is about 1/3rd larger than the Dragon Isles. Perhaps if Blizzard hired a few more environment artists, they could release a remade Kalimdor. Though, IMO, I would rather go visit brand new lands with new stuff like Pandaria with the Pandaren and Mogu and Mantid and Saurok, or the Shadowlands, rather than revisit boring old lands. I've already seen Orcs and Quillboar and Centaurs and treants too many times.
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Though another issue with an old world revamp (assuming the size was the same) would be how immersion breaking it would be to hop on your dragonriding mount and soar from one capital city to another in 20 seconds. It would totally kill any pretense of you living on a massive continent. The continent's size would have to be multiplied by 10x just to get us back to the current time it takes to fly from one city to another with old flying mounts, and even then it still feels ridiculous. The map would have to be 50x to 100x larger to make flying between cities at 800% speed feel like you might vaguely be flying across a continent to different kingdoms and countries. Maybe adding an hourly day/night cycle like in GW2 or FFXIV would help sell that sense of travelling across a continent too, rather than visiting everywhere in a single afternoon.