Personally, I looooove glass catfish. I've always wanted to keep them, but I don't have a big enough tank. You need to keep them in large shoals, and I think they're a bit sensitive to water quality.
How abouy Kuhlie Loaches? There's another fish I want badly. You also need to keep these in groups. They're really cute.
Tetras are always fun, too. I love Black Neon and Glowlight tetras.
White Clouds are really hardy and fun little fish. Cool thing about them is that they can live in both cooler and warmer waters!