1. It's simply a combo of items that aren't part of an outfit yet. A few ideas:
Tube dress + fishnet stockings + something else???
Legendary hard rock hat + very heavy metal belt (+ snakeskin pants?) + ???
3. It contains a new item that nobody has found yet, nor has been announced as available.
Maybe someone (myself if I find the time) can make a list of all items in the game that are NOT part of an outfit yet and see if we can see a connection or just start trial-and-erroring stuff until we stumble upon it.
Maybe if Gobb was feeling generous he could tell us which equipment slots are used for this particular outfit. (Hint, hint)
Any other ideas from anyone??