I would work on a amazonian theme using just fish and decor from the amazon region. It would have lots of tall upright peices of bogwood at the back of the tank to simulate tree stumps along a flooded river bank with smaller pieces toward the middle and foreground to replicate broken off pieces that have been washed down the river bed, a few large flat pebbles and pieces of slate at the front of the tank set into the sand substrate would finish of the decor.
For the fish i would start with a school of 3 to 5 pike characins (Boulengerella or Ctenolucius species, probably Ctenolucius hujeta) for the surface, a shoal of at least 6 silver dollars or related species for the middle and a medium sized predatory catfish like 3 to 5 Pimelodus pictus, a pair of Sorubim lima, a pair of Pimelodus ornatus or a single Pimelodus albofasciatus to prowl around the bottom of the tank. Then with the building block fish in place i would choose one or two "star" fish, a medium sized semi aggressive cichlid could work here or a oddball like a black ghost knife fish or one or two of the larger more brightly coloured characin species like Leporanis affinis or Chalceus macrolepidotus (or a combination of some or all dependeing on the size and temperment of the cichlid, Leporanis could not live with a BGK as they can be fin nippers). Pike cichlids (Crenicichla species) are a particualar favourite of mine and one of the smaller species (under 10") could work well in this style of tank.
Finally i would add a small clean up crew of hardy catfish like a couple of medium sized plecs and a few doradids (raphael catfish, spotted or striped) or Hoplosternum.