Improving the look of rivers making them bigger, deeper and give them their own feel with different fish you can only find in rivers. Making rivers a micro biome that is modular fitting into any other biome. Adding Small streams that flow into rivers could be cool and having the river be more realistic so they start somewhere at a spring and flow and join until they reach the ocean. One type of spring Players could find could be hot springs which could have an advantage perhaps you could find sulfur there which could be used With charcoal to craft gunpowder. Another really cool feature could be river deltas with mud that you sink into and farming crops at the delta would make them grow faster.