You could do a pistol shrimp and then get some kind of watchman goby. They will form a little acquaintance and dig a burrow where they will both live.
For a starfish your options are very limited. A chocolate chip starfish would work but it would eat all of the other invertebrates and coral in the tank. Asterina starfish would also work but can have weird shapes and are really tiny. Also breeds very rapidly. A brittle starfish would work, but some people find them creepy and they don't move like other starfish. A brittle star would probably be your best bet and they help clean the sand too!
Then you could do hermit crabs and snails and just have a little invertebrate community!
