Hello,
I have a suggestion for a more reallistic game. Currently the sea is a natural barrier for terrestrial units, but the countries are all flat lands. I think it would be nice if the main natural barriers (mountain ranges, wide rivers) could not be crossed by terrestrial units (or with movement penalty). This would make Alps, Ands, Himalaya, Urals, Amazonas river, etc, more difficult to cross. The strategy will change as to conquer Swizerland or Italy, you need air units.
Regards