Hey there,

Mistlands isn't out yet. The patch that's currently on the Public Test Branch is looking at the save system again.

Here's the info on new content from the pinned FAQ thread:
"Will new content be added to my world I currently are playing on?

New content such as tar pits, mountain caves and Mistlands in the future will not appear in areas that have been already generated. Areas are generated 300-500 meters from a player's location. "Exploremap" and "resetmap" has no effect."

Hope this helps!