Well it’s 2021 and SL is a failure. By 2022 subs will be at an all time low.
But I love the premise of your question. I have thoughts…
1 – Accept the fact that you are going to anger a lot of your playerbase but that you will deliver a better product and that 80% of those you lose will return and you will more than make up the difference with a better product.
2 – Blizzard as game developers have been conflating systems with content. They are focussed on expansion or patch specific player power systems that are aimed at forcing players to progress a certain way and time gate power gain. They have lost sight of what an MMORPG is – a big sandbox that we can play in the world with our friends.
Create content not systems. Focus on more quests, world content, dungeons, raids.
The player base does not want to grind out their toolkit through a convoluted essences or soulbind system to get to full power. They want their full character ability set to play with and then to get more powerful by progressing through content and getting gear. With that in mind, bring back Tier Sets that modify gameplay and provide player power that is then replaced purely by ilevel.
Every patch brings a zone (or an opened up / modified zone), a new dungeon, and a new raid.
Don’t try to be ‘cute’ with content. There has only been one success in new game modes in the last few expansions and that was the Mage Tower. Literally everything else has been a flop (Garrisons, Mission Tables, Azerite, Islands, Warfronts, Covenants, Torghast, the Maw), stop trying to be clever, just polish the simple until it shines.
3 – Stop trying to separate the player base. Horde / Alliance is meaningless after joining forces to stop so many raid bosses especially in MoP / WoD / Legion and then leaving Azeroth to adventure in the Shadowlands.
The Covenants in Shadowlands meant 3/4 of the playerbase never saw the quests and content you made. That is bad design.
You already allow merc-mode in PvP. Just remove the split. One story. One big bad threat. One faction (Thrall has the gravitas to pull that off). You can still be The Horde or the Alliance, but you are also (for example) The Grand Army of the Light in Legion, and the Legion of Maw Walkers in Shadowlands.
4 – Strip back from 13 playable races (and 10 allied races) to 9 races. Human - Dwarf - Draenei - Worgen - Orc - Tauren - Troll - Undead - Elf (with Elves to identify as either Alliance or Horde - not that it matters). Make all the allied races sub-race cosmetic differences obtained through character customisation (i.e. you can look like a Nightborne or a Night Elf or a Void Elf or a Blood Elf).
5 – Strip back from 36 specs to 24. Two specs per class (except Druid) and no class can do both the tanking and healing role (expect Druid). This involves a lot of deletion - including of Demon Hunters - but we need to swallow that whinging from the player base.
Warrior – Prot and Arms – Raid Buffs are Attack Power (Battle Shout) and Raid HP CD (Commanding Shout)
Priest – Holy and Shadow – Raid Buffs are Spell Power (Mystic Power) and Raid DR CD (PW Barrier)
Paladin – Holy and Retribution – Brings an Immunity (Divine Protection) and a HP Buff (Blessing of Fortitude) and Raid DR (Devotion Aura)
Mage – Frost and Fire – Brings an Immunity (Ice Block) Physical Damage Amplification (Armor Vulnerability) and Greater Invisibility (Party Wide Shroud)
Rogue – Assassination and Combat – Brings an Immunity (Cloak of Shadows) and Magic Damage Amplification (a Poison effect) and Shroud of Concealment
Hunter – MM and BM – Brings an Immunity (Aspect of the Turtle) and one of Raid Speed CD (Talent Modified Aspect of the Cheetah) or Raid DR CD (Talent Roar of Sacrifice)
Warlock – Affliction and Demonology – Battle Resurrection and Cookies and Gateway
Shaman – Resto and Enhance – Brings an Immunity (Elemental Shield) and Raid DR CD (Earthen Wall) and the ability to Ankh without taking up a Battle Resurrection.
Death Knight – Blood and Unholy – Brings a Battle Resurrection and a Raid DR CD (Dampening Zone)
Monk – Brewmaster and Windwalker – Brings an Immunity (Astral Dream) and a Raid Speed CD (Ancestral Winds)
Druid – Guardian and Resto and Feral and Balance – Battle Resurrection and a Raid Speed CD (Stampeding Roar)
In addition:
I would also remove Bloodlust/Heroism/Time Warp/Ancient Hysteria and make it a raid consumable only (Drums) to be added to Feasts and Cauldrons as optional buffs in organised content.
Engineering battle rez item is also gone.
Weapons are just one item (like Legion Artifacts). No farming for extra slots for dual wielders or sword and board.
6 – Strip professions back to three and you can have any two. Gathering Crafting Enchanting. All players get fishing and cooking.
7 – Completely separate PvP and PvE talent trees and ability tuning. This necessitates the removal of open world PvP but so be it. It’s one faction now, PvP is opt-in through BGs and Arenas, you no longer get to gank people in the open world.
8 – Ditch completely the push for mission tables and a mobile tie-in app. It is a terrible idea and a waste, utterly of developer time and resources. They are not useful and just shoe-horned in.
I think if drastic action isn’t taken, the game will die (or rather Blizzard will kill it) as it groans under the weight of nearly 20 years of design choices that no longer hold up.