Appearance. We spend our time and money to shift it, mold it, perfect it.
1) We know making new armor models is difficult, because they must all fit together, work with emotes, and other such factors. In fact, we pretty well won't be seeing any more.
2) At the same time, there exists many forms which, put simply, we would like to wear. For the assassin, as an example, both the cutthroats from Togo's Story and Anton's garb would be magnificent, but very difficult to render in the five-part PC armor method.
3) We can, however, mimic such forms. With Nightfall, Anet revealed the disguise ability, an effect which changes your model to look like something else. We can see its effect in the missions where you wear kournan armor, or in the transmogrification tonics which turn you into an elf or candy corn being. And, of course, dervishes use them all the time to turn into avatars.
Therefore, I propose a new class of item, the Guise.
Like a miniature, these items would sit in your inventory, take up one slot, and can be activated or deactivated as you wish. When using a guise, your model is replaced by the specified form. Using the aforementioned example, your assassin completes some goal that gives him access to the Nameless Assassin Guise. (Yes, the name needs work.) So, he opens up his inventory, double clicks the figure, and voila, takes on that nifty appearance for himself. It's already animated, it needs no new model work, and it looks quite cool. All would rely on the disguise function, which is already present in the game.
Guises could be from any of the various humanoids we see in the game, as they all seem to be using the standard animation templates. So, you could march with the Kournans, contemplate with the Kurzick, lounge with the Lionguard, or just jaunt with the Jade Brotherhood. None of them, however, would change any attributes, monster behavior, or otherwise shift game balance. The enemies will still try to kill you for taking their shoes.

I'll leave acquiring such items, trading them, using or disallowing them in towns, and other concepts for later. Simply put, does this idea sound like something you would want to see in Guild Wars?