World of Warcraft plans to add unobtainable transmog appearances in the future
Items are a big part of playing World of Warcraft. Sure, dropping ancient elves and demons and dragons to 0 health is its own reward, but half of the time they just teleport away anyhow to provide a future antagonist. It's the stuff you loot from the experience that really matters. The latest of the game's live Q&A sessions covered itemization, and you can either watch the whole video now or catch up with a summary. Keep in mind that the Q&A also specifically mentions not knowing when patch 7.2 is coming, so if you're looking for that, you're going to be disappointed.
If there are appearances in the game that you know are still unobtainable by players, rest assured that the developers are aware of it; finishing up the set transmog UI has made it clear what elements still can't be grabbed by players and need to be added. Legendary effects that feel substantial enough for a given spec are also in place specifically so that they can serve as a "test" for adding the effect as a baseline for the spec in the future. For the time being, the developers want to keep the 6-item sets that allow for three different tier set bonuses at once, but that may be changed with time. It's an interesting look at the game's current item design philosophy, although we make no promises that you'll be happy with the results.