Always fun to speculate about a world revamp and its implementation, here its my take:
- Current world and content remains unchanged, accessible through Caverns of Time / Portals / Similar system.
- Whatever continents are revamped are integrated as if they were a new continent / world, meaning that when you look at your map you will see the revamped Azeroth map and another one that leads you to "Old Azeroth", similar as how we can access, for example, the Shadowlands map in current WoW.
- I see a lot of discussion regarding a new leveling campaign and Chromie time. IMO Chromie time would remain as it is now for veteran players, and new players would play the new leveling campaign of a revamped Azeroth, which of course, would also be the campaign for max level players. You can make ONE campaign for both. Players who are yet to reach max level could keep leveling through WQs while they gain reputation for current content, while at the same time they keep improving their gear.
- A revamped Azeroth would be made from zero, as Draenor was, and of course some zones could be mixed so that they do not have to make 30-40 zones. Plus many zones could be saved for future patches / expansions.
- Chances are that if they make a new revamp It will be the last, so they better do It as evergreen as possible.
- The main campaign could be centered around whatever the new threath/s is/are (I put my money in several cosmic forces invading Azeroth, Yrel for the Light, Xal'atath for the Void, etc.) while at the same time seeing the progress of the different characters / races / factions in the world, introducing new characters and expanding all this through hundreds of side quests.
- Re-introduce archeology and make It fun while also useful tying It with old content. Give It a cool talent tree that makes us, for example, move faster in old dungeons / raids, chance for extra loot in old dungeons / raids, its own currency for pets, transmogs, mounts, etc.
- Expand the Dragonriding system to other mounts (pretty sure that they are going to do this anyway). Aquatic and ground mounts would have their own talent tree and all current colors, armors, skins... Would be implemented into one transmogrification system equal to what Dragons have in DF. Mounts numbers will be drastically reduced and their related achievements would be reduced in consequence.
- Return of the glyph system for skill customization, changing the color / effects / cast of some spells.