not a whole lot more...catfish..3-4 cories...giant danios as dithers or a couple of smaller gouramis.....in a 30 gallon i have red parrotts with serpa tetras with no problems...
I agree I wouldn't add to much else. The bottom is free so I would stick to adding fish there. Some of the mid sized catfish are a nice choice or else some of the smaller plecos might be nice as well.