Originally posted by Jarrod:

I'm not entirely sure how the generation works in this game, but something tells me they kept the mistlands in so they can populate the biomes in an update later and just add the content to them by re-generating those biomes.

That makes sense, but we don't know it'll work out in practice. When tar pits were added to the plains in H&H people needed to roll new worlds or use the Upgrade World mod to recreate that area. I certainly wouldn't build anything elaborate in any future biomes.