Navigable rivers would be great, but not for CV use. They would have to be unreasonably wide to allow passage, and sitting ducks while in transit. Now for river warfare, they would be great! We had a Rhine map someone built that got used in the Axis&Allies arena. There were PT spawn points on the river, and opposing v-bases right on the river banks. This would allow cross-river assaults. Cools stuff.
I'm not sure why we don't have river bridges in the MA. It would be cool to have a spawn point about a mile from the bridge, with the v-base just on the other side.