This'll never happen but what if every tier of every expansion was just treated like a sequential phased quest chain? (with anything time-gated removed)
Player kills Ragnaros, BWL is instantly available to that toon.
Player kills Nefarian, ZG + world dragons are instantly available to that toon.
Player kills Hakkar, AQ20 and AQ40 are instantly available with no war effort.
...
Player kills Kel'Thuzad, player can now go through the Dark Portal. When they do they get patch 2.4 talents BUT if they enter a vanilla dungeon they scale down to 60 and go back to vanilla talents.
...
Player kills Kil'Jaeden and can now board a boat to Northrend
...all the way to Shadowlands
The gear could also work like how transmog works, where you unlock a piece and then can summon it. So people playing BC could attend their guild's weekly Naxx raid to help guildies unlock BC, or hell, even just run it for fun and maybe unlock a few more pieces!
World-first players would obviously crush this but the race would be so GD long it would still take months. Weekly lockouts would still be an issue. They could get through vanilla probably in a few weeks but 2-3 weeks later they'd be trying to do something like Archimonde with extremely limited gear and it just wouldn't work. And there's no way to do splits because the process of leveling an alt through every freaking raid would take weeks or months!
Even casual guilds might have fun because the raid content would be so mixed up...a casual guild would maybe do a BWL/AQ40 farm on Tuesday, then Karazhan group 1 on Wednesday, Gruul with the tryhards from an allied guild on Thursday, and then the following week could be a Naxx full clear Tues/Weds/Thurs to get a group through to BC. Hell if a guild was big enough, months in to the game there could be something like Team 1 doing Cataclysm, then using their stockpiled gear to help Team 2 finish off Lich King, then the next week crushing Naxx to help Team 4-5 move on...
It would also open up some things that never happened in the actual game. For example, the world-first guild to get Sunwell done would have a gimmie that they'd also get world-first Naxx/OS/Malygos clear...so then like 24 hours after clearing Sunwell they're already in the middle of Ulduar trying to do s--- like Mimiron Firefighter or Freya + 3 in one week's worth of Naxx gear. It would make for some GREAT streams, it would be a LOT of fun to watch the world's best players trying to do these fights wearing rags.
So questions:
--Would you have fun?
--How long until Battle for Azeroth would get cleared?
--How would you handle the different difficulty levels?
--To accomplish the absurdly difficult task of getting 8+ versions of the game working seamlessly, how long would we have to chain Elon Musk, Brent Spiner, and Bill Nye to a desk in order to get this done?