Along the years there have been many hints pointing at the coming of N'Zoth. I've collected all of these hints into a single post. This doesn't proof anything, though, but I did it just for the funzies.1) In the Blank Scroll it's stated: The Dark Portal, left unguarded, was reclaimed by the Burning Legion. Horrors rose from the sea and found no champions left to stop them. So, after the Legion invades Azeroth, which is now, the horrors, N'Zoth and the naga, will rise from the sea.2) The last time N'Zoth took his chance was during the War of the Ancients. There hasn't been a Legion invasion of that size, until now. This could very well mean that N'Zoth will attack when we are at our weakest, fighting the Burning Legion.3) Xal'atath [the shadow priest artifact] whispers: The God of the Deep writhes in his prison, breaking free ever so slowly. You should hurry and defeat the fallen titan... there are greater battles yet to fight. Another hint at N'Zoth breaking from his prison very soon. After we've defeated the fallen titan in Legion.4) It was speculated that N'Zoth's prison is actually in Thal'Dranath, the island that will come in a patch during Legion.5) Nobbel once speculated that the appearance of the next Old God has always been two expansions after it was hinted by another, in a patch. C'Thun came in a patch during Vanilla, and hinted the coming of the God of Death. Then, Yogg-Saron came in a patch during WotLK, hinting the coming of Y'Shaarj. Then, we fought Y'Shaarj (kinda) in a patch during MoP. And guess what he said? He said that we'll rest in Ny'alotha. Which is related to N'Zoth. (more from this here.) Well, Legion is not obviously old god-related, but, as with all the other times the old gods have made their appearance, N'Zoth could very well come in a patch. And since there are currently only two patches confirmed, and one is Thal'Dranath, which was already speculated to be the place where N'Zoth lies...6) BFD was updated in WoD and along with it came some speculation. The imminent arrival of destruction. Hmm, who could that be? Either Sargeras, or N'Zoth. Or why not both? Also during WoD Cho'Gall (I think?) hints that N'Zoth is coming.7) There has been speculation that Wrathion is actually connected to N'Zoth. Well, Wrathion made an appearance in the Highmountain zone in Legion but was promptly removed for an unknown reason. Could it be?8) This is just my own theory that isn't backed up by anything, really, but I believe expansions in WoW are starting to follow some kind of cycle. Basically, there are 4 expansions in a cycle. The first always has to do with an alien planet. The second involves Dalaran and a new hero class. The third is caused because of N'Zoth and has a world revamp. And the fourth is set on a strange new land with a neutral race and class. TBC and WoD were both set on Draenor, hence being the first expansions of the cycle. WotLK and Legion are the second expansions. The third expansions are Cata and the next expansion after Legion. Because N'Zoth is it's cause, it's pretty obvious that N'Zoth will be the big enemy in it. Also, if it's Great Seas as has been long speculated, it could involve a world revamp with maybe some zones being flooded. The fourth expansion is MoP.9) (edit) The Nagas and the Emerald Nightmare are in Legion, which are both corrupted* by N'Zoth. There's also a new sentient bug race in Suramar, and so far all of the old gods have come with their own bug races, so it could be created by N'Zoth as well. (edit) I also forgot to mention that Neltharion's Lair, which is where Deathwing was corrupted, is located on the Broken Isles.*better?TL;DR: N'Zoth is coming soon, in either the Thal'Dranath patch of Legion or he will have his own expansion after Legion. Or, he might appear in Thal'Dranath and lead to the next expansion, like Gul'Dan did with WoD and Legion.