Oooh, lots of nice things you can do with that. I might think of trying:
Asian biotope with khuulie loaches on the bottom and a nice-sixed school of rasboras on top. Doesn't have to be harlequins, there are lots of other kinds. OR make that a school of pearl danios, instead- stunning-looking fish when kept right, though you wouldn't believe it when you see them at Pets@.Or, if you want something more unusual, there's those Asian rummy-noses (sawbwa resplendens).
South American tank with nice school of corys at the bottom and small school of phantom tetras and school of smaller tetras (like glowlights).
Central American with a group of unusual poeciliids- priapella intermedia or black limias or pygmy swordtails, with a slightly inauthentic bristlenose at the bottom, or another small loricariid.
Tanganyikan shelldwellers tank- and you could probably fit in a small group of (wholly inauthentic) goodeids at the top.
Pair of Bolivian rams at the bottom, school of black neons in the middle and a group of hatchet fish at the top.
Dwarf neon rainbows.
Of course, all the above setups do not assume the same water parameters- you would want to look into your local water and think about how much you are prepared to do about that.
