Arthas: Rise of the Lich King was released on April 21, 2009. Here's a quote from the end of the book when Ner'zhul shows Arthas visions of the future:
"…Something was roiling just beneath the ocean’s surface. The hitherto-smooth surface began to churn wildly, seething, as if from a storm, although the day was clear. A horrible sound that Arthas only dimly recognized as laughter assaulted his ears, along with the screaming of a world wrenched from its proper place, hauled upward to face the light of day it had not seen in uncounted centuries…."
After Cataclysm was released, people assumed that this particular vision was about Deathwing's emergence into Azeroth, but consider the following:
Cataclysm was announced on August 21, 2009. That's several months after the book was released. During the initial panels, Blizzard revealed that Deathwing emerged from the "Deathwing Scar" to the north of Stormwind. His point of entry was changed to the Maelstrom much later during the open beta. That's almost a year after the book came out. So it's pretty clear that Ner'zhul's vision is not about Deathwing at all.
Now, let's dissect Ner'zhul's vision:
"A horrible sound that Arthas only dimly recognized as laughter" can only be N'Zoth. An Old God's laughter is obviously going to sound very alien. Check Yogg-Saron's laugh and compare it with Deathwing's laugh. Yogg's is much, much, much more disturbing.
But the really interesting passage is "the screaming of a world wrenched from its proper place, hauled upward to face the light of day it had not seen in uncounted centuries…." Here's my interpretation:
1) "A world" is actually the Naga's underwater kingdom. N'Zoth raises it to the surface for whatever reason and the Naga are obviously distressed, hence the "screaming". You could say that Nazjatar will "face the light of day it had not seen in uncounted centuries…."
2) "A world" is literally Azeroth. Assuming that Azeroth was born when the Void and the Light initially clashed (see Chronicles pt. 1 for more info), this passage can be interpreted as N'Zoth doing something to rip Azeroth out of the physical universe and drag it closer to its birthplace. Azeroth could end up somewhere on the border between the Light and the Void, hence it will "face the light of day it had not seen in uncounted centuries…."
IMO the first interpretation is way more likely and people have been speculating for quite a while now that we would get Nazjatar as an xpack. What are your thoughts?