they should have had The Jailer be Tichondrius the Dreadlord leader and Shadowlands be the Dreadlord homeworld where he's usurped the flow of souls for whatever reason.
While your idea would of course be better than whatever random thing Danuser throws darts at, they already exploded the Dreadlord homeworld (Nathreza) in Legion prologue stories. Illidan invaded their homeworld with his Illdari, stole the Sargerite Keystone and used it to open portals all over the place that fractured the planet (kind of what happened to Draenor to become Outland when Ner'zhul opened too many portals.)
Also I believe we killed Tichondrius for good during Legion, and since they created the rule that killing demons in a fel-saturated area makes them permadead (IE : Broken Shore, Argus, etc...), I guess he's gone too.
I'd say they painted themselves in a corner with this, but then again it's pretty obvious Legion WAS the last chapter of the Legion and the overall story started since Warcraft III, and this Shadowlands retcon bullshit is just trying to squeeze more fluids out of the dead horse.
After all the Old God emphasis in BFA, N'zoth invading the world in several zones, prophecies of the Old Gods returning, this REALLY should have been a Black Empire-focused expansion instead of this cosmic undeath bullshit that lacks creativity (Here's Heavenland, Doomland, Castlevanialand and Nightelfland, so original). Imagine N'zoth reaching deep enough into the world through the Silithus Wound to reach the core of Azeroth and gaining enough energy to bring back the other Old Gods that were dormant. You'd have more zones invaded (a good way to remake some older zones that need upgrades, like they remade Arathi entirely in BFA for instance, but with always the choice between phases through bronze dragon npcs). You'd have players going back to existing continents so you don't need a whole new continent to develop, instead just remake some zones or add new ones since you'd have Yogg-Saron reemerging from Ulduar in Northrend, C'thun come out of Ahn'Qiraj, you could have N'zoth emerge from Nazjatar, and Y'shaarj would remain dead but you'd have new Sha emerging from Pandaria. This would make a good mix of various atmospheres since you could have icy environments, warm environments, lush environments, with a huge bestiary of corrupted void creatures, and endless possibilities to modify zones with the Old God architecture we barely got a glimpse of in ONE RAID. You'd have finally the possibily to create the honest-to-god Azjol'Nerub they scrapped during WotLK development, to act as a hub for players and the nerubian rebels that resist the will of Yogg-Saron... you'd have maybe the new Mantid Empress trying to resist the call of the Sha and we have to help the mantids, though they have their own sub-faction of fanatics that would attack us. The Qiraji would be all evil but you could have another war effort to shut the Scarab Gate again and a remake of the Ahn'Qiraj raid entirely remade in a new raid dungeon with new assets and bosses that would lead to kill C'thun for good with a new McGuffin that makes him really dead we swear this time. I'm sure Xal'atath (or whatever it's called, the shadow priest dagger / Old God chick) would also play an important part into the overarching story after all the hints and clues.
Shadowlands will always feel to me like the missed opportunity after BFA's foundations.