if you were going to make it into a marine tank, it's a bit small and i think you still need all the equipment you would for a big marine tank but i duno.
Theres plenty of choice, but whether you'd be able to get it is a different matter. Theres loads of mini killies, wild bettas, pygmy cories, other mini catfish, a trio of ottos perhaps, mini badis species, pygmy rasoras, pygmy sunfish, bumblebee gobies....