Basicly what the title says.
We can hide our helmet and cloak, but not our shoulders. But there are some tmog sets - mostly classic - that just look better without shoulders (like http://www.wowtransmog.com/armor/cloth/sages-cloth-set or http://www.wowmogcompanion.com/sets/embrace-viper).
Or give us the option to buy invisible green Shoulders. That would require no new menu option, so less work for devs and lets us just use tmog scrolls. I would even pay coins for that.