But Azshara does get my vote, because it can actually have a little bit of everything in it.
We've got the Tomb of Sargeras, home of many demonic relics, which can be the basis for a Legion plot. There's some kind of advance force seeking to use the Tomb to summon the full armies of the Legion onto Azeroth.
Then there's N'Zoth, and the Emerald Dream, which would likely show up in a Isle of Thunder/Firelands-type role in the expansion. As well as potentially having Ny'alotha appear as a zone.
And of course, we have the big baddie Azshara. Who's status as most powerful mage ever likely has only grown since she kidnapped the Elemental Lord of Water, Neptulon.
- - - Updated - - -
They need to put giant robots in as part of a mini-game like battle pets, everyone gets their own giant robot that can be upgraded and leveled then used in giant robot pvp and fighting against giant sea monsters. The only downside is that we would need an explanation as to why you wouldn't lead with summoning your giant robot in almost every fight situation.
Most people would rather die than think, and most people do. -Bertrand Russell
There is not a flag large enough to cover the shame of killing innocent people. -Howard Zinn
The stars may be far away, but people still reach for them. -My Grandfather
Reality is that which, when you stop believing in it, doesn't go away. -Philip K. Dick