That actually makes sense. We did see Gallywix negotiating with the Brokers in 9.1, so that is a plausible scenario.
I do think overall that Undermine is used is some capacity to locate the Dragon Isles. My guess would be that the isles are in some sort of time bubble and without the help of the Bronze Dragonflight or Kirin Tor, Wrathion goes to Undermine to get assistance from the Tinker's Union (perhaps Hobart Gapplehammer).
Within that premise, I can see Gallywix becoming a major antagonist. I still think Gallywix attacking Stormwind in a huge Godzilla-sized mech is still on the table. I don't see him as the final boss, but I do think he's going to be the final boss of a raid.
I also think we should finally see Gnomeregan retaken by the Gnomes and be a zone that players can actually visit like Ironforge or Stormwind. I would also connect Gnomeregan with Stormwind, Ironforge, Shadowforge, and Mechagon via a tram system. Undermine should connect to Gadgetzhan, Booty Bay, Rachet, a rebuilt Bilgewater Port (obviously), Plunder Isle, Orgrimmar, and maybe even Gnomeregan. And yeah, with Gadgetzhan's proximity to the Caverns of Time and it being a major Goblin land holding, I can see that location be a major part of the expansion as well. However, you're going to have to make it Hearthstone-sized;
And btw, if anything thinks that WoW doesn't incorporate stuff from Hearthstone, Blizzard incorporated Sir Finley Mrgglton and gave Hobart Grapplehammer his HS appearance.
Beyond the Goblin/Gnome stuff, I do agree that Void Dragons will be another antagonistic force, and I believe that they will be looking for the Dragon Isles as well, or have already found them. I don't agree that it would be Sinestra he leads them though, I believe it will either be a descendant of Vexonia, or Xal'natath.
They're going to need something major related to or on the Dragon Isles to have everyone chasing after it like this. You know, the standard Macguffin that gets the plot going. Overall, I do think that such an expansion would be a nice change of pace from Shadowlands.
Now this is the part I don't agree with. I do think the Scarlet Brotherhood will attack the new Undead under Callia, but I also believe that Turalyon will help defend them because Callia is Lightbound. I do agree with you that Anduin will be sidelined as he cares for his mental health after the events in Shadowlands. I also think that Gilneas will be returned to the Worgen, and we'll actually be able to venture there. Places like Southshore, Stratholme, Andorhol, and the Western/Eastern Plaguelands should be fully restored at this point.
#Team Steamscale.