The game creates zones as you travel in the game world. Usually in a certain range around a character. So any zone that all players haven't been near, will be generated when a player does get near.

How those zones are generated, is determined by the world seed. So when a new zone is discovered, a calculation is executed (they use a noise system for calculation) and determined biome type then. If Mistlands, it will then be the new design

Also, they might do something with existing discovered zones. They did that with plains last time, with the introduction of tar-pits. But with a large update as expected with Mistlands, it is to be seen.