First off I was against removal of the old zones to begin with. I hate removing content and also it made for a wonky levelling timeline. Blizz obviously didn’t care then so I doubt it would be a priority for them now but I wish they would fix it, it bothers me.

So if I was in charge I would probably massively expand scaling and have an opening scenario for every new character (skippable for old hands) which culminated in a tour of the portal room. In this opening scenario I would have the timeline explained, saying that the heroes journey began with the assault on the dark portal and journey to Outland, and then Northrend happened, and then Deathwing wrecked the world and there was chaos over all of Azeroth, then Pandaria, then Draenor, then Broken Isles, then Kul Tiras/Zandalar etc.

Everything below current content would scale because from a story standpoint with the content that’s in the game players should start in Outland. If people want to level from a story standpoint then cool, if they want to hit up their favourite zones also cool, if they want to do it one way with this char, another route with another char etc. also cool.

I think I’m on record as saying I almost wish there wasn’t an expansion with new zones, and instead old dangling story threads (like a lot of Cata zones) got resolved. So I would be really pleased if they added a couple of small new zones for current content but then most of it sent players all over Azeroth. I really love it when they do that, like with the class hall campaigns for instance, as it makes the world feel more alive. They could either phase these updates (or as the number of phased zones is getting to be a lot) they could utilise the caverns of time.

TLDR: I would update old zones by using them for current content and I would take the opportunity to fix the levelling timeline with a scenario/cinematic explaining it. So probably not all zones at once, similar to what they have done with Darkshore etc. I guess in that there are world quests and it’s very much part of the endgame.