To avoid being clickbait-y: my argument is going to be yes.
But let's get into it:
WoW, like basically every RPG that exists, from Final Fantasy to the Elder Scrolls, is heavily influenced by Dungeons & Dragons - a game you might have seen me posting a great deal about on this blog.
D&D's larger cosmology, which usually applies even if you're making your own homebrewed settings, is built on the idea of different planes. Your ordinary, mundane world is the Prime Material Plane - and in fact, all the mundane worlds where what we'd consider normal people come from (normal including elves and dwarves and the like) are in that same plane. There are a bunch of different planes as well, though, like the Inner Planes - which notably include the Elemental Planes - and the Outer Planes, which is where one tends to find gods, as well as angels, demons, and devils (in D&D, those latter two things are distinct from one another, though what you'd probably think of as demons in general in most settings are referred to as "fiends.")
But the planes the absolute closest to the prime material plane are two sort of reflections of the material world: the Feywild and the Shadowfell.
Now, obviously Warcraft has elemental planes (they even have distinctive names, where in D&D they're just "The Elemental Plane of Water," etc.) But we've also learned more about planes that feel very closely equivalent to the Feywild and the Shadowfell, namely the Emerald Dream and the Shadowlands.
Now, granted, the Feywild is much more like the land of Faerie as derived from Celtic mythology, being a place filled with magic and Fair Folk (aka Fairies) who act in ways totally foreign to normal people - meaning that they're just as likely to be murderously dangerous as they are to be genuinely helpful, and it's not going to be terribly obvious which way things will go based on appearance.
The Emerald Dream, outside of corruption by the Nightmare, is a much more obviously benevolent place, home to benevolent Wild Gods (which one could argue is actually more akin to D&D's Beastlands - an outer plane that exists between Neutral Good and Chaotic Good.)
If the Feywild is lush and filled with unpredictable magic and creatures, the Shadowfell is a bleak, grey reflection of the world that tends to be filled with undead and ruins - if you've ever played Dark Souls III, you could probably imagine most of the stuff in that game to look right at home in the Shadowfell. Actually, probably most of Bloodborne too. The popular Ravenloft gothic horror setting for D&D takes place in a group of pocket demiplanes called the Domains of Dread, which can be thought of as kind of soap bubbles within the greater Shadowfell.
In WoW, the Shadowlands serve a more specific mystical purpose - it's where the spirits of the dead go when people die. And whether it's a transitory plane before they go onto some afterlife (or are reincarnated) or if it's purely just the "underworld" hasn't been explicitly answered. Ironically, of all the planes in WoW, it's the one you've probably gone to the most frequently, given that our ghosts that pop in at the graveyard are technically traversing the Shadowlands.
Right: so would the Shadowlands work as a setting?
Basically all WoW expansions have primarily taken place in Warcraft's equivalent of the Prime Material plane. You could argue that Outland, being partially sunk into the Twisting Nether, makes it a planar adventure, and Draenor is of course in a different timeline (though potentially the same material plane.)
For many years, the Emerald Dream was considered an obvious location for an expansion - indeed, there was even work done on a large Emerald Dream area that found its way into Vanilla files - though whether this was meant to be a raid or an expansion I don't know. But Blizzard decided that the lore wouldn't make for a good expansion - not that there's not interesting stuff for the Dream, but that it just wouldn't be visually interesting.
The Emerald Dream is said to be a version of Azeroth that exists as if no civilization had ever risen. The Sundering never happened, and there's no buildings or settlements. So not a whole lot to work with for a whole expansion. Granted, we did see bits of Thunder Bluff in the Emerald Nightmare raid, so maybe that notion has been retconned. But it seems unlikely that we'll get that one.
But are the Shadowlands bound the same way?
In a rather pessimistic way, one can think of the Emerald Dream as the "before" picture of Azeroth. The Shadowlands, being the land of the dead, could almost be thought of as the "after." This is a place where civilization has arisen and all the changes the people of the world have wrought on the planet have taken place - but it's all in the past. Again, I'm reminded of Dark Souls III, where toward the end of the game (and a big chunk of the Ringed City DLC) you travel seemingly to the farthest possible future and see the many great civilizations' ruins all tumbling and crushing together, covered in ash. So you'd actually have plenty of opportunities to have interesting locations for us to visit in the Shadowlands.
But then there's the other issue: if we're literally in the Shadowlands when we travel as a ghost, shouldn't it really just look exactly like the real world?
See, in D&D, the Shadowfell isn't really where the dead go when they die - characters who die in D&D just die, and then they either get resurrected, or they go to the Outer Planes for their afterlife. The "ghost run" is really just a gameplay mechanic with some lore to explain it. In D&D, the Shadowfell is truly a different place - there's, for example, a city called Evernight in the Shadowfell that is the reflection of the prime material plane's city of Neverwinter, that, while a clear reflection of the other city, is fundamentally different, with its own inhabitants.
In WoW, you could imagine that the Shadowlands as it truly appears is not simply the ghost-version of the world. Maybe as a ghost whose time hasn't yet come, you don't see it as it really is. Or maybe, there's a "Border Shadowlands" and a "Deep Shadowlands," similar to how D&D has the Ethereal Plane (which, I should note, is where you tend to find ghosts, for the record) has a "border" aspect to it that overlays with the prime material, and a "deep" aspect that has you move sort of fifth-dimensionally into a misty world that has portals to the other inner planes. Perhaps the "Deep Shadowlands" could have its own places and even inhabitants.
Of course, alternatively, we could return to the connection with Ravenloft. The Domains of Dread are created by mysterious entities known as the "Dark Powers," who tend to find particularly evil (and usually tragically self-deluded) individuals and create demiplanes for them in which they are forced to live through their self-inflicted tragedies over and over, acting as both rulers of their domains but also prisoners within them.
This would be a way for the Shadowlands to have its cake and eat it too - being a reflection of Azeroth, but also allowing for cool new locations.
And you know why I think this would totally work? Because we've already seen one of these Warcraft Domains of Dread.
Helheim.
I don't remember if it's explicitly stated, but I strongly believe that Helheim is in the Shadowlands. But it's also not the equivalent space of anywhere on Azeroth (we know of.) And Helya, as a totally tragic figure who then did great damage to the world only to be trapped in her own realm totally fits as a Ravenloft-style Dark Lord (not to be confused with the Dark Powers.)
So one could imagine that a WoW expansion set in the Shadowlands could be a series of zones like Helheim, each with a different theme around the entities stuck there.
Thros, the land of Gorak Tul, was also not explicitly but almost certainly in the Shadowlands as well. Now, Thros did look exactly like the Crimson Forest, true, but it was also created for just a single (admittedly climactic) quest, and so I'd argue the reason it wasn't its own Helheim-like mini-zone was simply a question of developer efficiency.
I've generally assumed Ny'alotha is a place in the physical world - namely that it was simply the prison in which N'zoth was held (which would also mean that it was his capital city, like Ahn'qiraj for C'thun and possibly Ahn'Kahet for Yogg-Saron, assuming the latter really spanned all of what is now Northrend.) But given that N'zoth seems to be truly free of his bonds (and seemingly mobile - unless that was just his shadowy projection... I had previously assumed Old Gods couldn't really move given that they were burrowed into Azeroth) maybe the fact that Ilgynoth talked about going to Ny'alotha when we killed it, perhaps it's an Old God-y, void-corrupted area of the Shadowlands?
Given all these locations, and obviously with the potential for Blizzard to just make up more (I sure hadn't heard of Stormheim before Legion was announced,) I think you could very easily use the Shadowlands as a full expansion setting.
And frankly, given that I think WoW is usually best when it's going for the spooky horror stuff - between the Undead questing zones, Karazhan, practically all of Wrath, Black Rook Hold, and Drustvar - I could totally be down for an expansion in the spookiest place of all. Give me the Lich King, Helya, Bwonsamdi, and a role for Sylvanas that doesn't involve as much genocide, and I'm going to be a very happy WoW player.