My lfs breeds rays in a tank about that size. They have a second that is used for large trade ins such as adult oscars, midas, plecos and such, all seem happy to have the space.
I wouldn't put big fish like rays in it, I'd go for something smaller and active like a shoal of Tropheus and build it up with a bit of a rocky reef in the middle. It would make a great reef tank if you had the money! Colony of smaller pleco species, with a school of tetras and some dwarf cichlids? Upper jaw Polypterus species? Plenty of options really!