A harem of royal grammas would be pretty sweet...it's something I want to try eventually.
Another thing I've always wanted to do was a tank with like 20 different single species in it...all really colorful and various body shapes. I have a number of different fish in my 75g and it's a wonderful display at feeding time:
Flame hawk, royal gramma, a single female lyretail anthias, pink skunk clownfish, chromis, OSFF, starry blenny, etc....