Thought of this earlier, want to get some critique or thoughts before I put it on the official forums:
I think that they could use the new collections system to "unlock" various transmog items. Using the information from the Dungeon Journal to show all the possible drops from a boss, it would be possible to deposit items into the collection after you're done using them. Doing so would consume the item, but make it available account-wide for transmog. Obviously if you don't meet the requirements for said item, you can't use it, which precludes people wearing tier sets that don't belong to their class, with the obvious exception of the BMAH versions. This is also nice for when you can't seem to get the item you want on one particular character, even though it drops for your alts that can't use it. (Looking at you, Dragonscale-Encrusted Longblade.)
It gets the items out of your bag, and out of your void storage, while leaving more space for world-drop type sets. This wouldn't change transmog at all, but would simply make storage a bit easier as well as give you blanks to fill in. Plus it gives you a bit of an easier time if your particular class isn't good at a specific boss. (IE: Razorgore for pretty much everyone.)
It'd be nice if there was some UI that showed you tier sets or other sets that were designed as one, with info on where the item drops or is buyable from. You could maybe put in some achievements for completing specific tier sets, and have account-wide versions for completing every class for a specific tier. Feats of Strength for having complete versions of unobtainable sets, that sort of thing.
Let's hear your thoughts.