If we are talking really Ice Age then I would have though Abysians and C'tis would need to be drastically different as its just too cold for them. Abysia could live underneath the surface perhaps where rivers of Lava raise the temperature.
The nations should probably feel tribal rather than nations in the main. If they still do have something decribable as a nation then this could be a reflection of a very organised, disciplined, response to the Ice Age. I think you would want none of the nations to bear more than a passing similarity to present nations.
As the game will see the development of grand empires, not normally what happens in an Ice Age, it might make sense to set the game at the end of a period of Ice Age with whoevers God wins gets to decide the conditions in the next period - if its Caelum then its more Ice, Man, Pangaea etc then the greening of the world begins while if its abysia then the Lava rivers spout forth as volcanos light the horizens and the world is roasted beneath the fiery light of great sun.
Keir