With Druids of the Flame either killed or at our side - why not. Only way to our ED is from portals, or by help of enemy druids. And all druids rather dead or at our side.
Ofcourse some shenanigans can be made to break into capital, but same can be told about any capital or zone.

You could literally said same thing about Broken Isles after Frozen Throne. They could make continent even from banana island. How? Simple - as always 1 zone will be something you know, 3-4 zones will be something completely different. We have gone through this in TBC, Wrath, Legion.
Again, have some imagination people. Azeroth is not real world with its limitations.
Dont know if this has been posted already as I saw 25 more pages from where I left off but it looks like blizzcon goodies were anounced and the algarian stormrider looks to be an elemental.
Since they openly showed this I m guessing its not necessarily tied to the expansion? Unless I am mistaken and this is not the algarian stormrider.

It's easier to do that with islands since there's a lot more potential for variety. Undermine is completely underground and we already have a pretty good description of what it should look like. It's certainly big, but it's not big big. What you're proposing is comparable to turning Gnomeregan into the setting of an entire expansion.
That's not the Algarian Stormrider, it's the Bound Blizzard mount. The Stormrider comes with 11.0's heroic edition.
I thought so too at first since there's a similar-looking Stormrider NPC in the game, but they've already given the name for the mount shown in the image. It's called the Bound Blizzard, not the Algarian Stormrider.

That is a pattern you may wish to see but isn't backed by any facts.
I could go a whole different way that sounds as plausible, if not more so then the trilogy idea.
Example: Faction War + Old Gods is a common theme in WoW
Classic: Faction War stuff in Silithus + C'thun
WotLK: Faction War stuff after the Wrath Gate + Yogg'Saron
Cata: Faction War Stuff + N'zoth empowered Deathwing
MoP: Faction War + Y'sharj empowered Garrosh
BfA: Faction War + N'Zoth
This is such a common thread in WoW and goes through all Azeroth based Expansions. Only BC, WoD and SL had no real faction war + old god stuff.
DF breaks this mold a bit, we don't have any old god stuff currently, but seem to go more toward cosmic void instead of old gods, and faction war is, as of now, not a thing anymore.
They could have structured it so it is autumn when you enter the zone, and becomes steadily more winterized the closer you get to the center, and in the VERY center had the tiniest hint of spring (little flowers poking up through the snow or something like that). Or little spring themed pockets (but only a hint of spring as it is the death side of things) where they tend to the souls. Maybe the spring assets are only around the seed pod things when a spirit is about to be reborn.
It could have been a very beautiful zone that represented autumn and winter instead of being just another forest. It was a gorgeous zone but I find the aesthetic is better saved for night elves.
Probably not something that goes with your idea, i guess you have your own premise of something torghast like but i was thinking about corruption/nightmare like towers, without trash but with bosses on every floor.
Like let's imagine a scenario where Void(or x baddies) attacks certain lands and drop these big towers corrupting the land, that up to 3 players can enter(it scales if solo), you have to go through some floors to finally defeat the last one and make the tower disappear from the land in your phase, getting rewards at the end.
Kinda like Mortal kombat towers that have different fighter on each floor, with the baddest and hardest one in the end.