dwarf cories... panda cories.... maybe kuhli loaches... you have options
How about some nice black sand, a small piece of driftwood, some crypts and anubias along with 6 Otocinclus catfish.
This coming summer I am planning to upgrade 3 of my 4 tanks.
20 long to 40 long -
1 Black Moor
2 fantail Goldfish
I am planning to upgrade my betta to a 10 gallon and would like something for the bottom of the tank.
Side note : The goldfish actually came to me in the 2 gallon the betta girl is currently in.