I have a feeling it’s gonna be Lordaeron.
To be specific: Lordaeron Crypts raised as a floating Necropolis.
I think about something very niche every now and then: Back before WoW’s release, alpha concept art of the Eastern Kingdoms map labeled the Undead city as “Necropolis”, not the Undercity.
And based on its four-quadrant layout, it’s pretty clear to me that Undercity was originally meant to be a floating Necropolis like the Scourge used in WC3, aka a mini-Naxxramas. Death Knights took that concept over with Acherus but it’s not quite the same.
I recently started playing this character again after a massive hiatus from him, and the first thing I set out to do was get my Forsaken Heritage Armor. For those who haven’t done that questline it basically involves you working with the Desolate Council to empower plague eaters created by Faranell to cleanse Lordaeron of the blight, and you succeed. By the end of it the whole upper Lordaeron area is habitable by the Forsaken, and they talk about cleaning out the Undercity itself, but they never fully take it back, almost like there were ‘other plans’.
Since we know we’re going back to Northrend, and that Wrath was very centrally focused on Arthas, and the Forsaken have direct ties to him, I kinda think its fitting. As a Forsaken fan since pre-TBC, I have to admit, a “cavalry over the hill” moment where the Forsaken decide to raise Lordaeron’s crypts (aka Undercity) out of the ground as a floating Necropolis to unleash the ultimate Forsaken battle fortress would make my day. Sick as hell. And then it can just float over to Northrend.
Also I’m not saying we destroy the upper Lordaeron kingdom in the process and only pull out the crypts – I say we bring the entirety of Lordaeron with it. Revamp it like Silvermoon, combine upper Lordaeron with its crypts, make the whole Kingdom float to Northrend. Now we have a big restored Lordaeron Kingdom that everyone can use as their hub and Forsaken can have their Undercity back underneath it, and maybe there’s opportunity there to have a dungeon down there to clean out the last vestiges of the blight or something.
Plus now that Shadowlands is a thing the Forsaken can easily get some advisement from Maldraxxus on how to accomplish it.
I know this is extremely unlikely, and we already have a lot of good hub candidates (Ulduar, Wyrmrest Temple, maybe Icecrown Citadel or Azjol-Nerub since we’re probably going underground) but I’m biased and I love undead so there.