Everyone wants that, but I sincerely don't think it's possible, O.G. WOW was lightning in a bottle that came out in a time in most people's lives where we all had a lot of free time and everything was new, how in the hell do you recapture that? What exactly is it you're looking? I have logged in throughout the years to check it out and see if it can pull me back in and after the nostalgia quickly wares off your back to the same old grindy fetch quest game
Season of Discovery is capturing it quite well. The new quests, story content, and "remixed" abilities feel good while still having a good classic vibe to them all.
As for retail, it doesn't need to become Vanilla. It does need to go back to having unique and fun class identities with content that makes you feel like your class. It needs to go back to having races feeling unique with their own history and lore as the focus. It needs to go back to having strong differences in culture and identity between Horde and Alliance. It needs to not focus on designing its content around the 0.1% of "World First Raiders" and instead design content for the majority of its base. Even if that means that Mythic is made easier and Heroic is made harder. It needs to go back to creating a community that actively supports grouping and building an actually community.
Get rid of Addons. You should never be designing your content around the use of them.
Stop trying to make Mythic Raiding an "event" and just let players naturally go for world first. Even if that means world first is achieved within a few hours of launch. 99.9% of the player base won't be those who reach that.
In fact, lower the Mythic Raid number requirements. 25 was great when we had 11+ million subscribers. 20 was great when we had 7+ million. We are down to arguably 3-4 million. 15-10 would be preferable at this point.
Stop trying to remove class, racial, and faction identity. Instead add class quests back. Add reasons to go back to old zones and interact with your faction and races. The Heritage quests have been amazing. Do more of that, but expand it for factions and classes!
Make it so you have a reason to do older raids again. The previous raid in patch 11.0 should not be "dead" as soon as 11.1 comes out.