While the Burning Legion is cool, would you really want an entire expansion about them? Just looking at Tanaan Jungle in 6.2 bores me with all that green fiery gloom everywhere. No room for variety. I believe the Legion is best served in small doses, and the rest of the time it should simply be a looming threat beyond the horizon. Something you keep in mind, but don't need to engage with constantly.
I will agree and say that a cold landscape would be much welcome, however. I really wonder if there is a "Southrend".