I think there should be a tech to develop Mountain Paths ... most likely a tech in the Civilization era, near exploration, that has a greater likelyhood of arriving if you are near mountains. In it, you can build mountain passes, in certain areas, that only you can build. I think a Mountain pass would have to have pure mountain on either side (no parallel mountain passes), and have a movement penalty rather than a movement bonus (or simply 50% slower than the basic road). Only armies of nations that have researched mountain paths would be able to travel on such roads, and lowering the mountain into a plain would turn the Mountain Path into a "basic road".

Same goes for Thick Forests, imho (Forest paths).