Are you interested in making it in to a planted tank? That would look nice with a piece of driftwood and a splash of color, like maybe a couple of gold rams?
A pair of Cockatoo cichlids, or german rams,
or if you want unusual but not to expensive you could go slightly brackish with a purple spaghetti eel (Moringua raitaborua) that reachs about 18" max but is very thin and flexible, or a single nigerian sleeper (Dormitator lebretonis) both of these are often carried by Frank's Aquarium. The sleeper is currently $5. The pics are also from Frank's.
The peppermint pikehead, a subspecies of the pikehead (Luciocephalus pulcher), they get 6-7", Here are pics of both, the speckled one is the peppermint variety.
One site calls them the Frankenbetta.
bb cats are nice but they tend to be elusive great for collectors but boring for most.
I would have to agree with guppy on a pair of german rams in a nicely planted tank they are great also give you some care challange and are great to watch.