In my opinion, the ice is just there to cover up part of the map that they don't want to reveal yet. First of all, the ice looks very unprofessionally placed. Just thrown over top of it in Photoshop. The edges of the ice just seem to end in random places, like the middle of Skyarrow Bridge (which would probably be collapsed if it was really frozen), and at the edge of the desert (which a very small portion of the ice overlaps on). Also, note how the ice is very translucent. You can see water, grass, sand, and even a few buildings underneath it, including Dragonspiral Tower, and if you look closely, you can kind of see Professor Juniper's lab. All this suggests to me that, rather than Unova being frozen over, the ice is doing no more than just concealing new features of Unova that have yet to be revealed.