So... Alleria outright tells General Steelstrike that Xal'atath is -not- an Old God. That she's something "Else".
We also know that four of the old gods were imprisoned, according to the lore from Chronicles and in game... but. Y'shaarj was never imprisoned. Y'shaarj was killed. Aman'thul reached down and yanked Y'shaarj out by the stack and swivel and nearly killed Azeroth in the process, creating what would become the Well of Eternity.
So there are, or were, four old gods that were imprisoned. Meaning there's an old god under the Isle of Dorn/Khaz Algar, in a surprise to no one whatsoever.
But we also know that Eonar, ever the life-experimenter of the Pantheon, planted a World Tree on Khaz Algar. Her Keeper, Freya, is the one who messed with the waters of the Well of Eternity to create Scholazar Basin, Un'goro Crater, and the Vale of Eternal Blossoms, to give you an idea of how big on Life Eonar and her followers, like the Freysworn of Khaz Algar, are.
That she planted a World Tree from a gift given to her by Elune, whom she loves.
One that Aman'thul tore out. Why? "This is not order, this is chaos." was his response, but Life isn't Fel or Chaos on the chart so why use that imprecise language unless he thinks of everything that isn't Order as being Chaos? And why plant one right in the middle of the Isle of Dorn, near a tunnel that's so forbidden for Earthen they were programmed to fear it? That when some Watchers went nuts and started abusing or somehow harming their Earthen their Keeper decommissioned them by collapsing the tunnel on top of one of them?
The Earthen had long guarded that area of the Isle of Dorn. We know because there's an abandoned tower full of rusty weapons, there, that the Earthen who guarded it sought to use on the fallen watcher as they had on her partner, before her. What did the watchers -do- to deserve that? What is in that tunnel? And why are there Freysworn on the Isle of Dorn..?
My thought is: Eonar (also Elune and Freya) sought to 'Corrupt' the (currently unnamed) Old God with Life. Planting the world tree right into its body with the intention of changing its very essence with the power of Life.
Just as Eonnar is a world-soul turned Life Titan. She sought to make a Life Old God. Because she likes to experiment.
As we learn in Palawltar's Codex of Dimensional Structure in Hallowfall, mono-polar energy types are unstable and according to Ogden's law they bind to one or more secondary elements. It's why there's so much magma and lava in the Firelands (Fire/Earth) and the Arathi use Sacred Flame (Light/Fire).
But surely that -can't- work, right? After all, Yogg Saron corrupted Andrassil and destroyed it. And planted the seed of the Emerald Nightmare. And spread the Curse of Flesh to make the Iron Vrykul into flesh and blood. So Void and Life are incompatible, obviously!
Except...
Yogg Saron was the "God of Death" by his own claims. And the "Black Blood of Yogg Saron" is the basis of Saronite. It -had- been spread far and wide under Northrend and even into the Eastern Kingdoms and Kalimdor 'til Fandral Staghelm stopped its spread. (Tainted areas included Ashenvale, Duskwood, Feralas, and The Hinterlands) But this Old God's Black Blood isn't creating weapons exactly... it's turning Nerubians into Super-Soldiers.
Maybe Yogg was tainted by Death, just as Eonar is tainted by Life. Maybe the death in him is what made Andrassil something he could rot away? Maybe the death in him is what allowed him to corrupt the Emerald Nightmare?
Maybe this unnamed Old God, the fourth of the imprisoned Old Gods, didn't have Death in him. And now has life from Eonar's efforts.
In the bas relief mural we see of him in the City of Threads he's just a big ol' ball of tentacles, like Ultross from Final Fantasy 6. You know what else looks like a bunch of tentacles in Azj-Kahet?
The Roots. The roots that the Harronir, who look an awful lot like furry Dark Trolls, do so much to protect. They're also -obsessed- with staying hidden from everyone... why?
I think the Old God got infected with Life and bonded to the roots of Elun'ahir. I think the Rootlands are going to be part of the 11.05 or 11.07 patch, MAYBE 11.1. And I think we'll learn a ton about the Harronir when we go there. And how they're servants of this Old God, just as the Earthen and Vrykul were servants of the Titans. How every Troll and Elf on Azeroth is a "Child" of this Elun'ahir infused Old God.
How they're hiding from the Earthen and have been for millennia as they protected this one hidden Old God. Because they believe, however falsely or accurately, that the Earthen will try to kill them and destroy the last imprisoned Old God.
The Old God of Life.
Is it any wonder, then, that the Trolls would carry forth the druidic traditions of their Harronir forebears? That the Kal'dorei, even twisted as they are by the magic of the Well of Eternity, would still hold onto that ancient truth of their heritage?
That only by -abandoning- Life Magic do the Quel'dorei, Shal'dorei, and later Sin'dorei truly master the arcane magics they're so well known for?
And with knowledge of their new, very powerful, connection between Elf and Troll and Elune and Old God and Harronir... the elven tribes would seek to reunite?
That'd be a hell of a twist, right? The revelations of the Rootlands, of the -beginnings- of Troll and thus Elf-Kind, resulted in an attempt at a permanent peace between all the trolls and elves at a council of Quel'thalas...
Especially now that we've seen Elune's "Dark Face". That she might, in fact, be a Life-Entity touched by Void...