I agree, it sounds very cool. You could even do three:
River Bridge, Mountain Pass, and Isthmus.
Amphibious units should be able to go across water though, so that does create some extra work for correct implementation. And what happens if both sides have amphibious units? They should be able to fight in the water. Also amphibious units should not be affected by missiles when in the water, and if they are on fire then perhaps the fire should be extinguished when they enter the water.
Feel free to ignore these complications if you like
.