There's a whole range of items out there that show things on your back that aren't weapons or shields, the aforementioned keg, the Jet Pack from the ship fight in ICC, the gas mask from Love is in the air all jump to mind, but you can't really use them as apart of any armor set. And yet the back socket isn't called "Cape" or "cloak" it's "back".
Why haven't back items branched out in all of this time? Why can't we have jet packs, kegs, permanent back mounted-shields, flags, spine-protectors? Capes, frankly, suck, and there are all of two general models: a tiny bib, and the longer thin cape. Where are the huge flowing capes that, and this is the first thing that comes to mind, you would see Spawn or Batman wearing?