If H&H is the way they do it then... if you can "see" a complete area on your map (no fog of war) then nothing new turns up.. i had not explored any mountains except the edge of biome just showing on map.. after H&H dropped i later explored mountains and discovered Eggs and Moder Altar in areas i had edge mapped. but not earlier explored... so whereever mistlands will be on your map is a place you havent seen on map. So maybe dont go all Dora the Explorer to soon :-)