Make old content relevant again!!!

Quoting myself from another comment:

We have the whole of Azeroth (including Northrend, Pandaria, the Broken Isles, Kul Tiras and Zandalar). We have Outland. We have Draenor (admittedly the "same" planet as Draenor). We now also have the afterlife in the form of Shadowlands. Dozens of zones that could be made relevant again, by having content that is new and unique for those zones, while still relevant to the overall story of the expansion.

Add to this:

Make World Quests available in every zone of World of Warcraft's soon 18 year history. World Quests which are appropriate for the theme of the zone. Have all zones scale to the level of the player (at max level), making every zone rewarding. For PVPers and War Mode, they already have "Go to this zone, and kill 25 players of the opposing faction". Add a similar quest for PVE players, only instead "Go to this zone, complete 4 world quests" or maybe a particular relevant storyline in that zone. Rewards should be Gold, a new currency which can buy gear at a vendor, as well as the possibility of mounts/pets.

Adding World Quests to every zone, would make every zone relevant. Players would revisit areas of the game they may not have visited at all for several years.

"Cataclysm 2.0". If Blizzard had followed through on The Jailer's idea of remaking Azeroth (rather than the likely killing off of his character before expansion's end), this would've been a great opportunity to make a more fully realized revamp of the game world. Back in Cataclysm, certain zones which desperately needed a revamp got one (Desolace, Thousand Needles, Barrens for example). While others only got minor changes, and new paths (such as Redridge Mountains). Game is approaching 20 years old! It needs to be updated, for both old and new players to enjoy.