Sometimes, the light of the moon is a key to other spaces. I've found a place where, for a night or two, the streets curve in unfamiliar ways. If I walk here, I might find insight, or I might be touched by madness.
Ah, but what if they're one and the same?
I think the Rootlands will be just that, the Roots of Elun'ahir that reached down to the Worldsoul. The zone will be rootgrowth where the harronir live, surrounding Titan facilities (the Worldcore, the 11.2 raid), with Black Blood infesting large swathes everywhere.
What world tree? The tree was chopped. It's just roots.
I was thinking about how the crystals of calcified worldsoul energy might be scattered around the world if there are several the same size as Beledar, and I realised that if there are five of them placed around ancient Kalimdor then we roughly end up with Khaz Algar, Quel'Thalas, the Azuremyst Isles, Northrend, and a currently empty area off the coast of Stranglethorn, ostensibly the location of Plunder Isle and/or Hiji. That would create a pathway for the story to take us eventually to Northrend and can explain the inclusion of the Azuremyst Isles in Midnight.
Plunder Isle/Hiji could slot in as a minor patch somewhere, playing a major role in the story but not necessarily with a large amount of content. We could use some more threads for the Nightsquall story if not its full release. Alternatively, it's always possible that Siren Isle is playing this role and is located off the coast of Stranglethorn. I haven't played on the PTR so I'm not sure how significant the crystal there is meant to be.
In the vein of my earlier post about TLT, here's my predictions on how Midnight could go in terms of zones, dungeons, and raids:
Midnight
-Season 1-
Ghostlands
-ZA Void Incursion - Voidsworn forces are attempting to conquer the region where the capitol of Zul'Aman once stood.
-Deatholme - The undead Scourge remnants still pollute the area.
Amani Lands - East of the Ghostlands, expanded forest troll domain
-ZA Catacombs - Scourge remnants are reanimating the Amani dead of generations, but disturb the buried faceless one Kith'ix as well
-Loa/Troll Dungeon - Amani loa battle the forces of the Void
Northeron Coast - West of Ghostlands, North of Plaguelands, coastal region where the Arathi Empire has landed its main fleet
-Arathi Landing - The Arathi Empire has come in force, and some are too trigger-happy
-Naga Stronghold - The naga have established a fortress on nearby coastal islands
Eversong
-Sunstrider Isle - Void-infested region targeting new generations of blood elf heroes.
-Silvermoon Outskirts - Formerly ruined regions of Silvermoon City, infested with Void-aligned withered.
---Occupied Silvermoon - Raid. The heart of the Void's occupation of Silvermoon City. Azshara and her naga have seized control.
-Season 2-
Isle of Quel'Danas - The Void incursion began here at the Sunwell. The Arathi Empire and the heroes gather for the final battle.
-Magister's Terrace - A stronghold for Voidsworn forces.
---The Voidwell Plateau - Raid. The Sunwell, corrupted into a portal leading into the Void.
-Season 3-
K'aresh Remnants - Fragments of ruined K'aresh, hanging in the Void. Xal'atath's master Dimensius awaits.
-Ethereal City - Once a beautiful city built by the Ethereals, now ruined and corrupted
---Fortress of Dimensius - Raid. The throne of Dimensius, Lord of the Void

I agree with most of this, except I would expect Northeron (or whatever else they use for the relatively unknown fourth launch zone) to be primarily Naga-centric, with far less Arathi focus. I don't even expect their full fleet to be a thing in the Saga, since the Hallowfall Expedition was already supposed to be the team they sent to fight in Renilash (which sounds like Midnight). I think meeting their Empire proper will be saved for after the Saga.
The Naga, on the other hand, fit into the "Elven unification" part of the story perfectly, and Azshara was teased as a major player in this story in the early hints we saw during Dragonflight. I'm expecting a pretty big role for them as wild cards (though hopefully as untrusted allies- Azshara deserves to be more than a wasted mid-expansion boss again, and it doesn't make sense for her to go right back to serving the Void after plotting to betray N'zoth and getting tortured in Ny'alotha. I'd rather see her just exploit the whole situation and end up in a much stronger position at the end of the Saga).
Also, speaking of the Elven unification, this (and most other predictions I've seen) doesn't really address the other Elves' role in all of this. Sure, Nightborne should naturally show up with Blood Elves since they're united by marriage, and the other Thalassian Elves are returning to defend their homes, but Night Elves are detached enough from all of this to need their own role if Metzen really meant all Elven tribes. I'd pair them with the Naga instead of the Arathi- they have a lot of history together, and it's a good chance to make up for Blizzard's decision to have completely unrelated characters like Jaina and Lor'themar face Azshara in BFA.
I definitely agree that Azshara will have a huge role - I anticipate her showing up early on as aligned with the Void, but not necessarily being dealt with early on. She mentioned in Ny'alotha that she intended to seize the 'true throne of power,' and I'm pretty sure that means she is going to try and 'usurp' the Worldsoul, essentially absorbing its power and becoming it - rather than the Worldsoul awakening to make Azeroth some kind of deity, Azshara will become the new deity. At least, that'll be her goal. I also expect the night elves to be focused on dealing with her due to their history, that will be their role in the story. Nightborne can bridge the gap between Night Elves and Blood/High Elves.
I also think that the Arathi Empire will be a major element for future expansions beyond the Worldsoul saga, but I definitely expect them to show up with major forces to fight the Void. For that Northeron zone I imagined, I would see it was mostly naga and arathi, since that seems like the most likely place both will approach Quel'thalas from. Quel'thalas is their homeland as well though, they are descended from both elven and humans, so their presence would actually be part of the theme of elven unification.
There could be a major moment of the Hallowfall Expedition getting in contact with the Empire and raising the alarm that not only is Renilash here, but it's far worse than anticipated, essentially calling for back up. I think it's interesting to play the Naga against the Hallowfall Arathi though due to their experience with the Kobyss. They may be more than willing to lead the vanguard on them.
Absolutely agree on the Azshara claiming the world soul bit - I theorized with this during Dragonflight and got called an idiot in this very thread because I think she will personally believe she is above the void, and all other forces for that matter, and try to become born as a higher being through the world soul.
I guarantee they'll revamp Azuremyst Isles as well. If you think the Draenei won't get the spotlight in the Light vs. Void expansion you're insane. The Draenei heritage questline setup something I forgot in the Azuremyst Isles, and they'll want to revamp it to enable flying.
I was doing the blood elf heritage questline the other day and was reminded of the fact that the Shattered Sun Offensive was a coalition between the Aldor and the Scryers, and Quel'danas has always had a strong draenei presence in-game. Obviously they were a lot more united at the time given they were being assisted by the naaru to prevent an eredar from coming through the portal, but the Sunwell still has an innate connection to the draenei via M'uru.
I'd love a full Lordaeron revamp but I just don't see it happening right now. If the Azuremyst Isles don't get a revamp in Midnight, by the end of the expansion it'll be over 20 years old with no update or revisit of any kind.
given the roadmap: Nightfall=Hallowfall but permanently dark=stuff in the water might rise.
Though, i don't think they would make it permanently dark, but perhaps like the void incursions of BfA, a more extend, or phased version of Hallowfall.

I mean it kinda is isn't it? We don't know yet if thats the intended purpose of the beledar or if its just a side effect, but it keeps these creatures down in the undersea and away from hallowfall.
At the start of the expansion i thought the undersea would be a safe bet for patch content. Closing the beledar story and preparing azshara for midnight.
But with the worldcore reveal and undermine im not so sure if it still fits into the tww timeframe.
Also because i was just doing some transmog runs; Interestingly azshara is breeding those mindbender squids in the eternal palace. Never paid much attention to it, but with the reveal that they are intelligent and grow into ozumath sized kraken i wonder if this will be of significance.
Midnight dark waters can be terrifying.
“But this isn’t the end. I promise you, this is not the end, and we have to regroup and we have to continue to fight and continue to work day in and day out to create the better society for our children, for this world, for this country, that we know is possible.” ~~Jon Stewart