The devs have said that the three unfinished biomes will only get updated for areas of your map that haven't been explored so you might want to stay in the inner portion of the map for now. The mistlands only spawn far from center and the Ashlands/Deep North are at the polar ends of the map.

The devs have also said that they plan to have 9 bosses and do more with the ocean. So since the roadmap is out I'm gonna guess about the future content.
First we get Hearth and Home
After that we'll get the Mistlands with a boss
Then the Ocean with new ships and a Leviathan boss
Then the ashlands with Surtur?
Then finally the Deep North with I'm guessing Fenris as the final boss.