I think you should make it so that cities can't be built on tundra tiles. The Russian city spam is pretty ridiculous. Siberia as a whole should be a lot more sparce than it is now. In the case of important tundra areas (NE Canada, southern Alaska, and a couple of Russian cities come to mind), you can always put a single grassland so cities can be built there. I would also like to see jungles be unremovable except for when building plantations. It's a lot more realistic than having all the world's jungles reduced to giant swaths of pristine grassland.