Can I add more to a 29 gal biocube?

I have an established 29 gallon biocube which has been running fine for about 5 years. No problems or fatalities. It has live rock, a clown fish, a bullseye pistol shrimp, and several small hermit crabs and snails. Could this tank support more fish or inverts? Any suggestions that would work well with what I already have? I'm looking for something that will add some new interest to the tank. Thanks.