I’d bring back Arthas Menethil first of all–and keep him as a primary villain out there in the game for the rest of its tenure (never, ever, kill off a good villain).
I’d restore 40-man raid content.
I’d remove Mythic difficulty across the board, and only have LFR, Normal, and Heroic.
I’d get rid of ALL achievements that require multiple people to complete, the only achievement that should matter is what you the player can accomplish.
I’d stop offering loot that gives out insanely high numbers so that a squish is necessary every expansion pack.
I’d put a cap on the auction house that’s way lower than what it is right now.
I’d offer flying on Day One of every expansion for free–we all paid for it in the Burning Crusade for 400g/5000g respectively.
I’d bring back the music team that composed World of Warcraft Classic and World of Warcraft The Burning Crusade. How is it that music from 2004-2007 is more atmospheric and engaging than the entire Battle for Azeroth suite? Don’t get me wrong, BfA has some great tunes, but the repetitiveness is insane when playing great tunes over vs. an immersion soundtrack–like Eastern Plaguelands original music, or Felwood’s original music, you get the picture. Drustvar’s music, although great and well composed, is extremely repetitive. Stormwind City’s music, although excellent and well composed, is extremely repetitive. We don’t need a FANFARE everytime we zone into the city. Now and then is okay, but why everytime?
I’d bring back the RPG elements of MMORPG that Classic and TBC had.
I’d get rid of War Mode and have PvE, PvP, RP-PvE, and RP-PvP servers again.
I’d get rid of CRZ.
I’d get rid of Sharding.
I’d get rid of Phasing in all forms.
I’d get rid of linked and cross realmed servers.
I’d merge the servers to suit the less than 1 million subs population we see today in North America. We don’t need hundreds of servers.
I’d require people to have to travel to a Battleground entrance to play in a Battleground, and offer a higher reward for their efforts.
I’d offer high rewards for engaging in World PvP, true World PvP, not instanced or phased.
There’s plenty more, but those are the basics.