Just coincidence I guess, those words are very generic when you're describing moods and landscapes.
Besides that, I think the next environment will be sort of Alpine style. It looks obvious to say "Hey, TM1 had Desert, Rally and Snow, and now TM2 has Canyon, Valley, and a Snow sequel", but my estimation is not based on that fact. I think a snowy and icy environment would be the best complement to the existing environments in TM2. From an optical point of view, such as the climatic and car handling points of view.
Now, since TM2 is in any meaning going a more "extreme" way than TM1, which means the blocks are more extreme, less "blocky", and the handling is more extreme and intends to create speed feeling and "action" while driving, I think the 3rd environment will go this way as well. Remember the snow track from NFS Porsche or NFS High Stakes? A bit like those. A steep and cliffy, snowy and icy environment.
So I'm giving it a shot for the third environments name:
Glacier.
(If this fails, other shots would be
Crest, or
Summit, but due to the naming scheme (Desert, Island, Bay, Coast, Snow (original name: Alpine), Rally (original name: Countryside), Canyon, Valley), I think Glacier is still the most fitting).
