When Warcraft: Chronicle came out, they mentioned the Shadowlands only briefly, basically showing up in their cosmic chart opposite the Emerald Dream just outside of "Reality."
Death, as a primal force, occupied one of the six major spots on the outside of the chart. I don't know whether the existence of the Light and the Void as the top and bottom of that chart are meant to be more significant than Order, Disorder, Life, and Death, but it does seem that Death has its own realm of existence in the Shadowlands.
As a D&D fan (albeit a relatively recent one - though I guess it's now been five years) this immediately recalled their planes of the Feywild and the Shadowfell. The former is your classic Faerie Otherworld, home to beautiful but treacherous archfey (who aren't so much good or evil as... playing with a different set of rules) as well as a lot of mischievous tricksters who might not appreciate how mortal you are. The Shadowfell is a dreary and dark realm where emotions and color are sapped, and the undead are very common - tending toward a sort of gothic horror reality without necessarily being inherently evil.
Cosmologically, though, the Shadowlands in WoW are closer to the Outer Planes. These are realms that are sort of physical manifestations of the game's iconic alignments (Lawful Good, Chaotic Neutral, Chaotic Evil, etc.) As sort of conceptual realms, the Outer Planes are home to most gods in its cosmos, and are also where the souls of the dead go when they die.
In fact, there's a whole campaign setting that takes place entirely within those outer planes called Planescape - where humanoids are categorized as "Planar" meaning that they're originally from the Outer Planes, "Petitioner" meaning they're the soul of a person who died and has been reborn in the Outer Planes to go to their afterlife, or "Prime" meaning someone from the Prime Material Plane who has traveled to the Outer Planes while still alive (and are treated like the dumb hicks of the multiverse by most Planars.)
What's interesting is that, given that most of the mortal souls that travel to the Shadowlands on death stay there (even if someone like Draka gets assignments that take her elsewhere) in many ways, the leaders of each realm there are arguably the really important gods of the Warcraft cosmos. You spend one lifetime in the Material World, but the rest of eternity in the Shadowlands.
But up until this point, the closest we've gotten to a true pantheon of deities was the Titans. Indeed, we've literally referred to the Titans as the Pantheon.
But the Eternals - the leaders of the various realms of the Shadowlands - are also, at one point, called a pantheon.
There are parallels between the Eternals and the Titans, perhaps most notably in the traitor amongst them.
Sargeras had been the paragon of the Titans, but when faced with the horror of the void, he went down a path of madness. When he rebelled, he struck down his fellow Titans, and it was unknown eons before the rest of the Pantheon was restored, and able to trap Sargeras within their Olympus-like Seat.
In the case of Zovaal, though, it looks as if his initial rebellion was a failure, leaving him trapped int he Maw.
We don't have any real idea of whether the Eternals are on the same level of power as the Titans. My suspicion is that they're meant to be of roughly equivalent power, though the Eternals are masters of the Shadowlands, while the Titans are powers either of "Reality" (i.e. the material plane) or some as-yet-undiscovered realm of the Arcane, as we're told that they are beings of Order, whose signature magic is Arcane.
One thing that also intrigues me is that we hear mentions of the First Ones in Shadowlands. Are the First Ones just another name for the Eternals? Or was there some group of creators who built the Shadowlands? If these were the Titans, that would seem to put them a step up from the Eternals.
Indeed, I'll be honest, I'm a little sad that we only got to see the Titans briefly in Legion. Built up through all of WoW's lifespan, we ultimately only got to see several of them sitting in chairs during the final boss fight. I still don't really know the difference between Norgannon, Golganneth, and Khaz'goroth.
I know that a lot of the stuff we're getting in the Shadowlands is just new lore - though at the same time, it's using elements that were introduced long ago. The fact that Mueh'zalla turns out to have been behind a lot of bad stuff that's been happening is pretty interesting, given that we've had that name since Zul'farrak. I also like how they managed to imply that the Val'kyr are a weirdly bastardized version of the Kyrians, which also manages to draw a connection between a Norse mythological being and a Greek word associated with Christianity.
Of course, if we're talking Gods, there's none more mysterious than Elune, who seems to be tied into a lot of things in Shadowlands, but also defies categorization, with deep connections to the Titans (Tears of Elune, Eonar found on "Elunaria") the Wild Gods (Cenarius is her son) and the Naaru (Khadgar claims a book in Karazhan says that she created the Naaru). Now, we've also got the Winter Queen possibly referring to her as her "sister" (though I'll admit there are other candidates for whom she's referring to - though not a ton of them) and once again I find myself wondering what the hell Elune is.
While a lot of Shadowlands is adding brand new lore to the Warcraft cosmos, not only is it really interesting and cool lore, but I also suspect we're going to get some major reveals that will resonate even after we leave this realm behind.