With all the Classic+ discussion, I am aiming with this post to create an alternative to merely listing things I hope for, but rather a description of what I think the game should have done in its progression. Hopefully this will shed light on future direction with classic servers now in operation.
WoW contemporaneously and historically suffers from design flaws stemming from a lack of vision by Blizzard. This includes parasitic game elements, unnecessary overhauls, graphical inconsistencies, and a mentality on pushing content out too quickly. This has exhausted the game's potential and we are witnessing a trend of marginally decreasing content quality and quantity around and in the game.
So to get into it, I have created an example timeline of WoW expansions with example content. The philosophy is that expansions should meet four criteria:
One, Expand on the existing core game
Two, Explore new content as fully as possible
Three, Be meta-thematically consistent
Four, Respect existing lore as much as possible
These can be summarized into WoW expansions necessarily respecting the base gameplay and base aesthetics.
Timeline:
World of Warcraft 2005-2010
The Burning Crusade 2010-2013
Wrath of the Lich King 2013-2017
The Roiling Tides 2017-2020
Rebirth of the Dragon 2020-2023
The basic timeline here requires a lore overhaul, with a summary below.
Vanilla carries mostly similarly. The Burning Legion stays more distant and manipulative - They are the end big bad. They become somewhat more real in Outland. Us in contact with the Legion again, Arthas re-awakens his Scourge to prepare for its inevitable invasion and we take the fight to Northrend. Whilst occupied in Northrend, Azeroth's over-extended kingdoms are losing grip to a series of minor powers that were previously in a state of weaker submission. Finally this culminates in magic-political chaos that manifests the Old Gods, and the Dragonflights intervene as their role dictates to restore some semblance of order.
This is the overall story arc specific to WoW. No world soul, void lords, First Ones that totally aren't Titans, etc. - These new cosmological roles are just endless rabbit holes that are not fitting WoW well.
Now assume existing iterations of the game have the generality of their content realized in this timeline as well. I will focus on additions or changes. This is not wholly comprehensive as that would be too long of a post. This implies and ignores some lore changes mentioned above.
Vanilla, 2005-2010
+New Zones: Hyjal, Uldum, Ahn Qiraj, Quel'Thalas, Gilneas, The Red Forest(Scarlet land Northeast of Plaguelands that's currently unused), The Highlands(Twilight Highlands but no Twilight).
+New Dungeons and Raid Areas: Karazhan, Dire Maul, The Scarlet Cathedral, The Caverns of Time, Silvermoon Citadel, Grim Batol, The Emerald Dream.
+Raid difficulty system: Raids are single difficulty, with some challenge based higher tier or extra rewards. New raids may be higher or lower tier difficulty than existing ones. Dungeons follow this logic too.
+Expanded Class and Race quests: Lore demands certain groups did certain acts, such as the Horde ultimately stopping the Blood Elf threat, the Alliance beats Karazhan. Classes and races get more racial lore and gameplay, specific armors, and flavors for their classes.
+Professions Expanded: Even greater use for professions with more recipes. Solo prpgression attainable through long and arduous crafting end-game.
+PvP Expanded: Gearing through various world objectives, and looting of other players (Not literally, the game would rarely make a Bloodforged copy of a player's gear to loot in honorable combat. Bloodforged gear can be redeemed based on honor rank and armor quality).
+Worldbuilding by Devs: As Vanilla goes on, the world is built up to finish being cohesive and breathing. Gnomeregan and Echo Isles become smaller racial capitals connected to Ironforge by Tram and Orgrimmar by Boat, respectively.
+Small QOL improvements: Trade orb terminals in every capital city allow you to trade to another connected player in another city so you may reside on your favorite hub.
+Language Secondary Skill: Learn your faction's languages and complete associated content with language requirements.
+Reputation Rework: Reputation is both a bar and set of requirements, such as learning the Dwarvish racial language before becoming exalted.
The Burning Crusade, 2010-2013
+High Elves and Goblins as playable races. Unique to them: High Elves are neutral and able to quest with and within Quel'Thalas, an otherwise Hostile faction. Goblins may trade cross faction and use a neutral auction house. High Elves start in Quel'Danas, Goblins in the Azuremyst Isle.
+Draenei are a sort of Pandaren in Outland; Neutral but have two faction reputations similar to Scryers and Aldor focusing around the Tribal and Holy wings of their society. They are native to Draenor but grew in numbers when the Orc society collapsed, and were shielded from much of the destruction. They are not spacegoats. They are why the Legion could not come back to Azeroth so quickly.
+New Raids integrate with the raids of Vanilla in the difficulty system. More raids and dungeons than normal TBC.
+Skill based flying and 150% speed ground mounts.
+World Challenges: Completing the Sunwell raid as a non High Elf allows neutral status in Quel'Thalas.
+Jewelcrafting allows for a neck socket that, with a class questline, may alter spell appearances and effects. Change your gem for different effects.
+PvP Expanded: More PvP rewards and objectives, such as capital raids, Gurubashi Arena objectives, etc.
+Level Cap Remains at 60: Completing certain expansion challenges similar to Pathfinder gives additional talent points in the existing(assume gameplay balance rework to the trees) Vanilla trees so players grow stronger by taking advantage of their class' full abilities. Power creep severely reduced and ultimately respects the core game.
+Additional low level content
+No Alliance Shaman, Horde Paladin
+Alt currencies paired with gold with their purchases. You cant buy your way to power but you MUST sink your gold. In themepark MMOs, devs should be actively trying to make players poor so they play the game, and so inflation does not skyrocket. More gold sinks.
End Raid: Sunwell
Wrath of the Lich King, 2013-2017
+Neutral Cities: New Heartglen, the Argent Dawn's main and massive Northrend base in Dragonblight. Dalaran bubble is opened as a neutral city and also allows portals back and forth.
+Death Knights are playable to Orcs, Undead, Humans, High Elves, Dwarves. Themed around third war, not new scourge invasion.
+All classes get 15 pathfinder talents and a fourth talent tree. I am a large fan of classic wows bring the class, not the specialization idea. Level cap 60.
+New Raids/Dungeon areas: Malrannas(Naxx can stay in EPL), Scarlet Harbor, Gundark, Niffelheim, Wyrmrest Temple, The Kaluak Hunt, Crystalsong World Bosses.
+PvP Raids: Instanced PvE raids with npcs that are more player-styled encounters where depending on the raid, npcs will cc heavily, be mobile and brutal, trash heavy for PvP rewards. Other raids will involve two competing faction teams that will at times fight one another.
+Inscription Expanded: Glyphs do more, from cosmetic to gameplay.
+Scourge Infection: Various temporary scourge infection and invasion events in Northrend settlements.
+Expanded Guild System: Guild housing, rewards. Rewards include achieved progress, and potential progress so small and large guilds alike have access. Players with all faction reputations exalted who are also in a guild may earn the title "The Diplomat" and gain the ability to speak the main language of the opposing faction.
+Player Cottages: Housing that allows displaying of achievements and very minor gameplay aspects (No AFKing in garrison). Highly customizable and players may gain multiple income/material generating estates in the world. Highly popular, RP-friendly, or niche-specific estates gain more. Account wide so choices matter and you dont have 50 income generating estates on your account.
+Continuation of Alt Currencies + Gold mechanic
+Survival Mode: Add challenge to your gameplay by partaking in Survival mode for increased rewards.
End Raid: ICC
The Roiling Tides, 2017-2020
+World update a bit but not a total conversion; Made for flying, which is still skill based.
+New Zones: Kul'Tiras, Tel'Abim, Maelstrom, Zandalar, Vash'jir Islands.
+New Raids and Dungeons, some focused on Azeroths kindgoms being challenged by smaller powers: Steamwheedle, Venture Co, Zandalar, etc.
+Level Cap Still 60, +10 talents.
+Talent rework to make it a bit more modern
+Archaeology Profession
+No new races but hordes of new character customization. (Allied races are a stupid concept imo)
+Very light amount of new race/class combos: Up until TBC your race was a very specific instance of a class(Ex: All paladins were Silver Hand knights)(Who remembers being shocked at Dwarf shamans when a shaman wasnt just any random nature elemental wielder but a specific, tribal type) Human/Gnome/Undead hunters, Troll Warlocks. This also helped balance the racial situation a little bit so it should be kept IMO.
+Player ships: Additional light housing option, where everything from rafts to battleships are available. Alliance boats, Horde Zeppelins. Boats move incredibly fast in water, faster than many flying mounts, Zeppelins offer highly safe travel that defends from air enemies that knock off fliers on normal flying mounts.
+Twinks Abound: Low level content added for more twink items, voluntary exp loss.
+Level Prestige: Prestige at 60 back down to level one where you can start with some abilities from higher levels, cosmetics, and gain new rewards.
+Class and Race Spirits: Combine characters by completing a questline to "die" unique to your class (warrior heroic battle, warlock sacrifical altar) and integrate abilities, cosmetics, or other bonuses such as a third professions onto another character. Useful if you like making alts but want to focus much energy on your main or want OP human racial (at this time) on a Tauren. Collect all spirits for powerful bonuses. Gain other race's racials, unlock unique race/class combos.
+Transmog tab and collection from day one, option to disable in PvP.
+Transmog competitions, and quests that rely on certain scores, or disguises such as breaking into Ravenholdt!
+Low level reforging
+Zone Invasions: Zones temporarily face unrest and chaos, where players prevent the poltical upheaval and Old God resurgence in the land.
End Raid: Azshara's Palace
Rebirth of the Dragon, 2020-2023
End Raid: Titan Watchers