Cool tank! How about some loaches? (yes, I love loaches!) Sewellia lineolta maybe or sidthimunki come to mind. Love the goby! & the shrimp looks great too!
I have river rocks from the great lakes but are you near the ocean? Lots of rock hunting possibilities if you are...It adds a fun dimension to a day at the beach. Also check landscape places, Home Depot-like etc.
Can't go wrong with java fern on wood or a clump of crypts near some rocks in an Asian themed river if you want more plants.