Bichirs! Always number one for interesting IMO.
Pantadon butterfly, ctenopomas, mystus catfish/sun catfish, smaller variety of eel, pleco variants, elephantnose.
Plenty to choose from although you may be restricted by the volume of the tank to how much stocking you would like. Bichirs would be fine so long as it's not an ornate. Ctenopomas are perfect, pantadons are fine, mystus cats only get to 15-20cm, sun cats potentially get too big, the smaller eel types reach 20-30cm (not fire eel/tyre track), plecos are all ok except for maybe really large sailfins, elephantnose get to 30cm.