- Boats without ice-breaking ability can't travel (and, possibly, may perish out of a safe bay);
- Land units use rivers as roads and may enter frozen water (maybe also with a danger);
- The map looks different! (In Wesnoth the view is darkened at night, idk if it is possible to set another effect)
Also, currently a temperature map is built by map generator but is destroyed after the terrain is placed, so if a polar tile has been once somehow transformed into jungle it has the same ability to stay as one as one on equator. Maybe we could have climate-dependent effect on convert time, and climate-dependent probabilities of GW/NW effective levels ("climate" means latitude plus local/adjacent terrain reqs), as well as aforementioned freezing-thawing happens not in all map (and, for a fine world, in different times in different hemitoroids).
Maybe the normal impact of GW/NW may become smoother, applied each turn to a few tiles rather than many tiles in rare turns.