My 10-gallon tank is themed to look like the Caribbean. I already have an underwater cave, sea shells, a skull (which my male betta uses for a home), a few e=sea creature decorations, rocks and an underwater cliff.
German blue rams have a mohawk like dorsal fin. Keep only 1 or 2(male/female) of them though. Not very sociable with their own kind. If you keep a pair be warned they breed like rabbits. I would say they are very punk looking.
Black angels would look cool - sharp and pointy.
Bright red swordtails might look good too, intense color with a sharp dagger tail!
I agree with the rams having a mohawk, it's just that they don't really have enough attitude..... they're not so tough.
Maybe a school of tiger barbs -- a good size school, they got the 'tude, the color scheme and are kinda Halloweenie....... did I just say weenie?