I've been wondering about the possibility of creating a functional St. Lawrence Seaway. Essentially a river that you can put boats on.
Currently, I believe that mapping water tiles diagonally makes them disconnected, so you would have to use a staircase pattern to make it sailable. It would be nice if there was some way to represent a long sailable canal with just diagonal tiles rather than a huge staircase pattern. The same could be used elsewhere in the world to add some interesting gameplay such as the canal that goes from the black sea into Russia, essentially giving Russia a bunch of cities with coastal access.
Maybe this could be rolled into the redesign for naval units. You could have a new unit that sails on rivers, or introduce a new fort that can be built on rivers and can be placed next to other forts, allowing you to make a fort chain for extended naval access, but only on rivers.