My money is on something like "Collector" UI or "Transmogrification Wardrobe". They've been hinting for quite some time that they're looking at other ways for collectors of different things to finally have a space to put them and possibly arrange them coherently, instead of just enlarging the bags more and more until the screen is entirely filled by bag slots and/or pages of bank space are introduced.
Yep. I think that would be a pretty big feature. Making a UI addition not unlike the Pet Page where you can see how far you are on collecting a certain set that might be a PvE tier set, or a completely made up by the player set. Also the loads and loads of vanity items that only take up bag space can get stacked in a nice page like that with a preview window and information on where to get it and all sorts of other interesting things.