I like the cyp/shellie idea. I'd even consider cyp/shellie/julies, with a string of rockpiles and interspersed shell beds. Add some bolbitis, java fern or crypts for a bit of planted color.
Or...Tropheus, seems like a good tank for that.
Or...a school of Geophagus with some cories and a couple of huge schools of big bodied tetras (black phantom, or columbian or any of the hyphessobrycons).
I like all the swimming length, but the narrowness does kind of stink. I'm enjoying a 5' tank with a group of midsize schooling fish like barbs/tetras with a bunch of cories on the bottom. They really take advantage of all the length and provide a ton of action to watch without seeming constricted by the 12" width.
Looking forward to seeing what you come up with.