Species tank for something or other . . . seahorses are really neat when they are cared for properly. So are mantis shrimp. (I am sure you knew that though)
Another idea depending on how much you want to spend is an anemone tank. I have seen some tanks that are devoted strictly to the condi and let me tell you . . . they were impressive!
I think that a species tank is the way to go, but you gotta come up with a cool theme.
Justin