Okay, I'm not putting any of the "leaks" that's been going on aground for a while; but lets take step by step plausible reasons as to why Dark Below is gonna happen.
Okay, 1st of there's always been a trademark registered, about 3 months before Blizzcon and it always came true. Not to mention there's been Blizz's attempts of "cover-ups" in the past expansions, like how they tried to cover-up goblins and worgen for Cata and pandaren for MoP.
And remember, Dark Below was registered before the Diablo 3 expansion, and after that it was followed up by a bunch of random fake registrations.
If Blizz will announce the new expansion during Blizzcon, which they will, than Dark Below is it.
Next bits will be regarding the lore reasons:
Since Cata's release there's been numerous tidbits about the 4th Old God that rests at the bottom of the sea, N'zoth.
Those of you who managed to put together Yogg-Saron's Puzzlebox, have heard countless quotes which direct to him and the place where he resides:
We know that it was he, with who Queen Azshara made a pact to transform her and the remaining Highborne into naga to save them from the Sundering.In the sunken city, he lays dreaming.
The drowned god's heart is black ice.
At the bottom of the ocean even light must die.
In the land of Ny'alotha there is only sleep.
Ny'alotha is a city of old, terrible, unnumbered crimes.
In the sleeping city of Ny'alotha walk only mad things.
The stars sweep chill currents that make men shiver in the dark...
The fish know all the secrets. They know the cold. They know the dark.
He helped Lady Naz'jar, under the command of Azshara, conquer Elemental plane of Water and kidnap Neptulon the Tide hunter, using the army of faceless and Ozumat, Fiend of the Dark Below.
Grim Batol, which is invaded by Twilight Hammer, is controlled by Erudax, Duke of Below.
It was N'zoth who corrupted Emerald Dream, and even tho it was pushed away, it still persists at it's source. And since the Rift in Mealstrom and Rift of Aln are connected, that's where the Old God rests, phased in 2 worlds.
Dreathwing was corrupted by N'zoth, as in Dawn of the Aspects, it can be seen that, before his transformation, Neltharion was already hearing Old God's voice. During the Well of Eternity, we see N'zoth's corruption protecting the disc from Deathwing. Lastly, during the Dragon Soul raid, we encounter 2 of N'zoth's servants Warlord Zon'ozz (who names his master A LOT) and Yor'sahj the Unsleeping, who are protecting Deathwing to ensure the Hour of Twilight happens.
And finally we have a speech from Ra-den in Throne of Thunder: But there is a yawning chasm of darkness beneath you mortals, vast, endless, and all consuming. I do not believe that you can correct this doomed course. But you have earned the right to try. Farewell.
Now before, it was assumed that he was referring the Y'Shaarj, but with Dark Below, it's more likely now that he was referring to N'zoth.
OKay, for the next bit, I want you to consider this, especially those who whine that Burning Legion and Sargaras should be in the next expansion.
I get that throughout the MoP, Wrathion kept constantly reminding us of impending return of the Burning Legion and that we must do whatever it takes to stop it from coming to Azeroth and winning. And of course, when things didn't go as he planned, he threw a tantrum and swore that he'll do whatever it takes to prepare for Burning Legion's arrival.
Now, if Burning Legion invasion was gonna happened so soon after Siege of Orgrimmar, than we have a problem.
1. There wouldn't be enough recuperation time for both Horde and Alliance to be ready for a full-blown demonic invasion.
2. I doubt even Wrathion, with all his now-titanic powers, could realize Plan B so quickly, it'll take time to concoct whatever his scheme will be.
Also consider this: Azshara and N'zoth are the 2 remaing evils in this world that still pose a great threat. So what chance do we stand against the greatest evil in entire universe, if we can't even handle our home problems (quoted from Loth'remar on Blood Elves joining the Horde)?
Last but most important: Anduin Wrynn.
All of you who read Prophet's Lesson, Velen's short story, after Anduin reminds him that even tho, Burning Legion is the biggest danger in the existance, that doesn't excuse him from ignoring his responsibilities and the need of people here and now; after which Velen receives a vision:
Anduin Vrynn will be the key player in the battle against the Burning Legion as Velen saw in his vision.And as soon as Anduin came into view, the world changed.
Instead of the prince, an armored warrior stood before him, his plate helm and breastplate shining with the essence of the Light itself. The warrior wielded a sword forged from the same material as the armor, held it aloft as he perched on an outcropping... Whether it was on another world or Azeroth, Velen couldn't quite tell. And suddenly the dark sky above erupted with the combined chivalry of Azeroth's races.
The blood elves, orcs, trolls, tauren, and even the accursed undead and scheming goblins rode flying mounts of every sort and description. They were armored and armed in magical weaponry glowing with so much power it hurt Velen's eyes to look at them. Beside the legions of the Horde, the ancient night elves charged with humans, dwarves, and gnomes, whose ancestors formed the original Alliance, and the shape-changing worgen were united with them. Velen's own draenei bolstered the army, their ranks adorned in otherworldly metals and bearing crystalline maces and swords.
The Alliance and Horde were not alone.
Dragons swooped and soared in formations that made the sky resemble a giant multicolored, reptilian wing. They blanketed the horizon with their physical size and numbers, and when they roared a challenge, it shook not just the earth Velen stood on, but the universe as well.
And yet, for all this, the greatest shock to Velen's senses came as he saw those that flew just beyond the army of dragons. The naaru had taken the field, so many that Velen didn't understand how creation could contain them. The power of these beings of Light filled Velen's heart with hope, swept away the lonely centuries, and left him in wonder that he could ever despair, that the dark, no matter how terrible, could ever truly reign.
And then a shadow fell.
It was vast and empty and swallowed all light that entered it. Velen knew it would be all consuming until, at last, it would turn and devour itself, endlessly gnawing on nothing in the Great Dark Beyond, removing all meaning from the universe, from the most heart-breaking sonata to the most arresting sunset. It was too terrible to see, to comprehend, and yet the army headed straight for it. And the light began to fade...
Standing before the Prophet was only a human man-child, eyes wide and passionate, saying something unintelligible to him.
All of you, who pay attention to lore and Anduin's travels across Pandaria, you'd note that after the "Garrosh-smash-puny-human-prince" moment, even tho Velen managed to heal his shattered bones, he still needs a walking stick to go aground. What Adnuin did gain over the course of his journey, is an expanded mind and sense of wisdom, which is something that Wrathion, the 2 year old whelp, failed to do. And his involvement served not just the Alliance, but Horde as well.
If the vision does show Anduin being a warrior of Light, while his connection to Light is as good as it gets, he still lacks stregnth, or otherwise physical capability. Meaning that Dark Below will put him through another development, physical one this time.
Since there has been showing defeat of Garrosh in Siege of Orgrimmar, there's something that caught my eye.
When Garrosh is defeated:
No, it cannot end like this. What I…what I have seen? Nooooo! This world, is my destiny! My, destiny.
Let me note that at that time, Garrosh absorbed the full corruption of Y'Shaarj's heart.
DO you remember another person who took into himself corruption from the "remains" of an Old God?
He's your answer: Cho'Gall.
In comic series, when Cho'Gall absorbed the power of C'thun's dead husk, he was not only imbued with his power, but also given the vision of the Old God. We mite be seeing something the same here.
WHat if all Old Gods on Azeroth are connected through a special link? Remember that all the Aquir cities (Ahn'Qurij, Azolj'Nerub and Dread Wass) are sister kingdoms. What if what Garrosh saw was an inevitable return of N'zoth?