I would like to see some sort of Hell. Where all the evils Link has ever banished reside. It would be full of re-deads and skeletal enemies. Poes and brand new demon enemies. It would have many craggy rock formations, lava spewing mounds...etc.
Link would be sent there to retrieve something...or for some other random reason. Maybe he falls through a portal that leads to there?
Anyway. Other than a hellish nightmare world, i'd like to see some form of ruins, atop a mountain...that floats. Like in the movie Avatar.
It wouldn't be covered in snow, just plain rock.
The ruins would overlook the entire overworld, which would consist of lusious forests and vast blue lakes.
The dungeon itself would be inside the mountain, and when Link completes it and is about to get the key plot item, the villain of the game would come in, take it and destroy the floating mountain, causing it to crash into the earth below.
They'd be my dream trerrains. The Crypt (as suggested by several members above) would also be great.