Time to whip out my “World of Warcraft 2 is never going to happen” stick. Lost track on how many times I told this to people.
I think you’re making the planning far harder than need be. For instance, any WoW “expansion” could be a full game overhaul if they decided to rework from the ground up. It’s just a new client / server software with the ability to import characters.
It would be the end of WoW officially on that client software, but the franchise would still continue.
Classic showed they had an inclination to separate server / client software with version number. Blizzard has said the technology could be used for any of the xpacs if there was demand - which is new. So they could do this if they wish.