After slowly introducing more and more ridiculous items over the years, Blizzard has now finally decided that the overly restrictive and downright archaic transmog rules are no longer necessary. In other words, you will soon have much greater freedom when it comes to customizing the appearance of your character's armor!
One of the biggest changes coming in with the next patch will be the ability to hide every single item except your pants, so if you're a fan of your Demon Hunter or Lightforged Draenei's tatoos, you will soon have a chance to show them off to the world. And if you simply must run around in your underwear like a maniac, worry not as there are plenty of transmog options that can give you the sort of look you desire.
There will also be a variety of improvements and additions to the transmog system, though Blizzard hasn't specified what exactly will be coming with the next update. From what I can tell, however, I think we're going to be able to transmog between all appropriate weapon types (maces -> swords for example), transmog our weapons and armor into some of the sillier models out there, as well as transmog into some of the cool grey gear that was previously inaccessible.
Once Blizzard reveals some concrete information I'll make sure to let you know, but until then I'm afraid all we can really do is sit and wait. On the positive side, at least we know that they are seriously working on improving the transmog system!