I'd fill it with sand and driftwood and get a nice big group of orange head geos, not that unusual but a tank that's raely done right and looks good when the fish are all interatacting and fully coloured.
Another suggestion if you want biotope style would be an Asian river tank with fast flow water and lots of loaches etc. bouncing around in the current. Add some rocks and plants and school of mid water swimmers of some kind and you've got a nice tank. You could also do a similar African set up with kribensis and Congo tetras and a few more odd ball fish from that part of the world. Lots of options for that size tank, have fun deciding.
Another suggestion if you want biotope style would be an Asian river tank with fast flow water and lots of loaches etc. bouncing around in the current. Add some rocks and plants and school of mid water swimmers of some kind and you've got a nice tank. You could also do a similar African set up with kribensis and Congo tetras and a few more odd ball fish from that part of the world. Lots of options for that size tank, have fun deciding.