Unfortunately I feel like we’re only going to see WoW2 when WoW is brutally killed off and left as a perpetual content-less MMO. We’re already seeing that with things like Mythic+ where using older content in a “cycle” helps turn old content into new.
But then I consider looking at Blizzard’s history of unceremonious decision making, by looking at HotS; by all metrics, Heroes of the Storm was growing in revenue, players, and viewership year by year. Sure, it wasn’t as big as League of Legends, but it was performing almost 10-15x better than Hi-Rez’s SMITE, (a that game is still running with plentiful content, crossovers, season passes, new Gods, and more to this day). In light of all of that information though, and despite HotS functionally being “a successful game” (and an extremely well designed one at that) by all relevant metrics, they still “killed” it at the end of 2018, ceasing all future content production and leaving a skeleton crew to finish the scraps before finally ringing the death knell for it last year.
Why? Why would Blizzard do this? It’s because in their eyes, it simply wasn’t making enough money to justify the effort. This is a company so wrought with fat-cat ideologies at the core that it doesn’t matter how passionate a dev team is if the people who call the real shots likely don’t even touch videogames and see it as just another business venture. You think the exec side of Acti-Blizz give a crud how long or loyal some of us players have been? That’s gonna be a tough reality check for some. (RIP HotS, you were done so dirty.)
In the case of the HotS story? The irony is that Overwatch 2 is and has been performing with around the same numbers as 2017-2018 HotS did, and with OW2, they jacked up prices to levels that skirt on unbelievably expensive. Is that that game’s final attempt at becoming “worth the time” as well?
One could argue WoW is on a similar path as HotS. One could argue it’s doing even worse if the unofficial measurements like achievement tracking, google trends, and other values are speaking truth.
All in all, WoW2’s likelihood will 100% hinge on WoW’s ability to continue success despite its age. Ask yourself- did WoW Classic only take so long because up until that point, they thought themselves better than needing to do it?. I do not for a second doubt this subtle “sunsetting” could very much be possible. I also suspect that it is in the minds of Blizzard and players alike that newer, upcoming MMO’s (like the Riot Games MMO, or newer entries like Phantasy Star New Genesis, Lost Ark, Ashes of Creation that all lookk visually gorgeous) are going to be sporting a 20+ year engine advantage and pose a major risk to WoW like no “wow-killer” ever has before.
At the end of the day, all we can do is speculate, but also use history to show that they won’t hesitate dropping us like a sack of 
the second WoW becomes that 0.1% less profitable than it needs to be by fat-cat standards.
All in all, if or when that happens, that’s when WoW2 will become a thing. When they’ve milked this female tauren dry to the point where age cannot be ignored, and MMO competition makes this game look like a fossil needing to be dug up by Archaeology.
My only request is that WoW2 is less of a ‘new game’ a more of a ‘next-gen remaster’. Think “we rebuilt the same game, but with an engine designed less like a house of cards, and more like a Lego Set”, where parts of the engine can be ‘phased out’ as newer tech appears. For those wondering what that looks like, Fortnite/Unreal engine is actually a perfect example. Because at the end of the day I think all of the 5 year+ players aren’t going to be too keen to see everything they’ve worked on get axed.