well a standard 15 gallon i believe has the same thick glass a 55 is made of. so im sure you could go rimless on a 15. its short enough to not bow. a 30g (or anything around there) cube may also be able to go rimless (current usa solana etc)
A flounder harem would be cool. I've always wanted keep a flounder among the many other things I've wanted.
Or you could get a bunch of corals that fluoresce a lot and put a black light over the tank. Instant party lighting! Maybe you could get some of those bioluminescent fish to put in it. ;] Or even glowing plankton would be cool.
I was actually talking with my hubby the other day about this idea: Getting a few large diadema/long spine sea urchins and a school of bangai cardinals and putting them in a display tank together. Bangai cardinals spend their whole lives living amongst the spines of diademas and you can just feed the urchins nori a few times per week. I have 2 good sized diademas that need a home if you want to try this. You could give the tank a black/white contemporary theme.
how about like an amazon river type thing with a river in the front and plants and land in the back u could do frogs,lizards, newts things like that would be ultra cool