I think an elephant nose may look nice in it, or a dolphin mormyrid, but the dolphin may outgrow the tank, a LFS has a large one in a 20 gallon and it seems incredibly cramped. Baby whales are cool, except for they stay under 5" and if you just put one in the tank would look very empty unless you have some other small fish with it. Remember mormyrids only eat live or frozen foods and are bottom dwellers so don't put any pig fish in the tank with them or else you will be spending a fortune on food.
African knifefish stay under 8" I believe. They look like a mix of a knifefish and a clown knife.
A school of glass catfish and maybe a few other peaceful fish could look cool too (especially xray tetras), except I never had luck with having calm looking glass cats.
You could put a single electric catfish in the tank, but it would outgrow the tank eventually and will need a 40br or a 55/75 gallon tank.
These are what I would want to put in a 30 gallon.