You're missing the point, you don't need to start over... you just need to discover NEW Mistlands biomes if you've been to the EMPTY Mistlands previously. Mistlands is not as common as Black Forest or the other biomes, and it only starts appearing pretty far from the center of the map.

Have you actually TRIED to go to Mistlands on your existing world? Or did you use a map reveal command and now you're mad at the devs because you ruined your map?