My Perfect WoW expansion would have:
1. The mob difficulty and gear rarity of Vanilla
2. The threat and mana management systems of WoTLK
3. Class quests and quest chains reintroduced, including class spell rewards.
4. The MoP talent system, with characters gaining a talent point every 10 levels.
5. The removal of several guild perks, including gold generation and XP buffs.
5. The WoD dungeon and raid mechanics, and a level cap of 100.
6. Guild Garrisons and Garrison Wars (guild V guild PvP)
7. All classes reworked to reclaim class identity but remove convolution (similar to Legion)
8. Nathrezeim and Valkyr as new hero races (unlockable through achievements)
9. A Mythic Dungeon system for every dungeon in the game at every level, up to Mythic 30
10. A Mythic Raiding system for every raid at every level (ex. Mythic Molten Core 15), up to Mythic 30
11. An expanded profession system.
12. Artefact Quests and Weapons acquirable only through Mythic Raiding.
13. Legendary Armor (sets) to replace MoP (world drop) Legendaries aquireable only through Mythic Raiding
14. Heirlooms and Heirloom Artefacts crafteable through professions. Recipes found in Mythic Dungeons.
15. All quests categorized as Class, Main Story, Reputation, Profession or Faction (PvP) quests and all others removed.
16. A Plethora of additional quality life changes (such as buff stacking) too numerous to list here.
I would name this expansion "World Of Warcraft: Tale Of the Titan", as it would be told from the perspective of Azeroth's Titan.