There are several shells hidden in the code, you can see them all at http://www.destinydb.com/search?q=shell but currently there's no way to access them. I know that some people glitched into a HoW area and they found a new shell on a dead ghost, but those who used it were supposedly banned. There's another Shell that came with special editions called "Frontier Shell", you can see it on any of my characters. EDIT: Added picture because I have nothing to do right now...