Personally, I think you could do an Mbuna colony. I think a 66-gallon tank is plenty of space. Peacocks and haps, though I wouldn't do in anything less than a 75.
You could also do some SA/CA cichlids like Convicts, a Blood parrot, EBAs etc.
Other ideas could be livebearers (easily a big enough tank for those) or a bichir tank with Senegals and some pure top dwellers like African Butterflyfish. Or an African biotope. Some oddball ideas could be doing a Red Wolffish (only fish in the tank though because of their aggression), a big crayfish tank, or a really fancy aquascape with some small tetras and angels, or a shrimp.
I hope this gives some ideas for what you want to do.
