For the base answer of what Shadowlands as a place is: I would take inspiration from Brandon Sanderson by turning it into a place where souls go to for a very short time that is being inhabited by all sorts of magical people, before passing on to the real afterlife that is left unknown. Said place in his books also happens to be a sort of strange reflection of the real world, as the Shadowlands has been portrayed before its expansion. An important part of the magic system involves the ability for people who have been powerful magically to linger for longer or (in some cases) to even leave behind a “shadow” that has their memories and personality (it is left unknown whether these shadows are actually the soul of the person in question or just a copy while the real soul has moved on to the actual, unknown afterlife).
The greatest narrative, universe-breaking problem of Shadowlands has been the fact that it is THE afterlife, turning the WoW universe into grimdark setting, especially with destruction of souls and Maw for everyone. If we turn it into a temporary place where souls go (with time there being possibly even in seconds for those who wish to progress further), we fix this problem.
At the same time, we can use Jailer without changing him a lot, as someone who has been doing all that he can to capture souls, preventing them from moving to the afterlife, and using their energy to fuel his armies which would not be the souls themselves but rather the denizens of Shadowlands. The Jailer´s armies would be a great threat to the other denizens because they are constructs, something that others can´t replicate, fueled by souls that have been put into a coma of sorts (not tortured by the Demons of the Warp…right, sorry, wrong universe).
The stakes? If Jailer wins this war of conquest, he will be able to fully control the Shadowlands and just snag all the souls, depriving them of their real afterlife. This is why we need to help the other denizens of Shadowlands in their fight against him. What is his motive? Does one need greater motive than more power and getting to paint the map with the main color on their empire´s flag?
Secondly, by keeping Shadowlands as reflection of Azeroth, we can limit the scope to our planet only (we could add Outland for good measure, explain it by it getting connected to Azeroth if we wanted to get us some folks who died in Outland). ALL OF CREATION isn´t getting threatened, only our little corner of the universe.
And, a cool little benefit of it being shadow of Azeroth? The zones could be reworks of existing ones, with some cool flair. I don´t want to go too much into this as the base zones aren´t really something that important in the grand scheme of things. For all I care, the base design of “not-Light”, “not-Emerald Dream”, “not-Plaguelands” and “not-Castlevania” could work fine (alright, maybe without not-Plaguelands). After Alliance and Horde have spent their forces, it´s up to neutral organizations to take the mantle. Argent Crusade goes to not-Bastion, Ebon Blade goes to not-Revendreth, Cenarion Circle goes to not-Ardenweald, and we could get some zone for Earthen Ring, perhaps some plains and stuff. Kirin Tor can be saved for Dragonflight, as I´m assuming that´s going to happen still.
Jailer, the Eternal Ones, these guys also don´t need to be some second pantheon. They could simply be powerful beings (with no need to explain their origin) that emerged in the Shadowlands and lead various denizens of this plane of existence, each having their own small empire being threatened by Jailer. Our role as a player could be to convince them to finally begrudgingly work together and our Covenant might be actually important storywise as, even though we helped each of them, our choice to ally ourselves with a rival empire and in turn getting barred from their seat of power would make much more sense.
That´s the base story, now for the patches: I think getting a foothold in Jailer´s home zone with not-Korthia could work fine, ending with us destroying Jailer´s empire. Zereth Mortis we would have to completely scrap and instead, we would get reworked Icecrown.
And now we come to the last and the toughest part of this all: Sylvanas
Good thing is that the whole “this world is a prison” schtick has only started with Shadowlands cinematic and is up to change. Since Cata, she has been all about getting future for the Forsaken. However, there is one massive issue with undead: They are, by nature, parasitic. They cannot procreate naturally, they need the corpses of others. However, since overwhelming majority of people on Azeroth are unlikely to willingly become undead (this includes their allies), they can only achieve this by force. Which, in turn, will leave the area they conquer without living inhabitants that could provide new corpses.
I would put this problem as the core motivation for Sylvanas in BfA and Shadowlands. How does she try to solve it in BfA? She sees the signs of aggression from the Alliance, emergence of Azerite, and decides that the war is going to come sooner or later. She might as well start it and win it.
Her plan to occupy Teldrassil is recontextualized as a way for her to get new corpses that she could raise without her allies becoming her enemies. Night elves would still be allowed to live, but their dead bodies would become free game. And sure, elves live long, they don´t really die that often, but the same goes for undead. She didn´t need million new Forsaken, she only needed enough to replace the infrequent losses of Forsaken life.
Of course, this didn´t work out. So, what could the new plan be? Maybe the bodies could be created and used to house souls. Anduin can´t really stop you from raising someone when you don´t need to But, where to get the souls? This is what got her to research Shadowlands and get in contact with the Jailer. They make a pact: Sylvanas will fuel the war, sending more souls for Jailer to capture and in turn, once he takes over the Shadowlands, he will start sending her some back to help keep Forsaken a thing.
But, it goes sideways and Sylvanas, in a last-ditch effort, rips open a way into Shadowlands (not in Icecrown) to help the Jailer with her Loyalists and, eventually, get her side of the bargain. However, throughout 9.0 and 9.1 she realizes he has never intended to do so.
So, as we defeat Jailer at the end of the raid, she steps in, draining what is left of his power and opening another rift away from the Shadowlands, escaping us again. And where does she go?
Her allies won´t allow their dead to be raised. Her plan to create some form of sustainable system for undead creation failed, because her enemies wouldn´t just accept brutal occupation (and Horde probably wouldn´t be too happy about the idea either in the long run anyway). Her plan to get souls and create sustainable system without the need for conquest failed too. She is left without allies.
But, there is a power out there which she is familiar with. An army that could take over the whole world and allow her to enforce her will on everyone. An army that she will have complete control of. No more allies betraying her, no more weak servants. She would be able to create the future for the undead as masters of the world and, with draining of Jailer´s powers, now has a way to contest its master.
The final zone and raid would be rework of Icecrown, in the real world, after Sylvanas became Lich Queen. We need to stop her before she can fully take over the Scourge (because Helm of Domination would probably take some time to master).
The expansion ends with Sylvanas finally defeated. At the end of her life, she realizes she has become the same monster Arthas did. With her last breath, she uses the remaining power gained from Jailer to release the souls of Scourge undead into the afterlife, finally removing the need for there to be a Lich King and finishing what she has started over a decade ago: Freeing the undead from the Scourge. There will never have to be another Lich King ever again.