The short term things can be done currently, while the long term things are something which I hope to see in the future
Ice caves
A number of caves which are very cold, and composed of snow tiles (which we already have). Frozen rivers should be common and walkable on (I don't think it'll take much effort to make a frozen tile), some of these caves could have secret areas which are reachable by melting a particular tile (using fire
Tropical Beaches
Beaches which have a line of sand touching the edge, then reach the water. Should have a few palm trees and (?? pine?) trees dotted about. Should be home to small creatures such as crabs, and should be very good areas for fishing.
Mountains
We have mountains, we have mountain passes, we have snow on the mountains, but where are the mountain goats and LLAMAS?
Well, that's all I could think of. Some of these suggestions can easily be done by our brilliant map team. The creatures ones obviously need to have graphics and the like bla bla bla