A concept discussed by many over the years:
A truce between the Horde and Alliance. Let each race have its own already existing home city instead of a faction, with palyers having the choice between which existing city will be their home.
Let Faction war cease, battlegrounds and arenas will still be functional however as well as opt-in world PvP, but instead of factions there will be guilds or wahtever other artificial seperation groups that particular character can join.
Ambassador system: Any character created on a faction can opt to visit any city or settelment without being attacked, if PvP is switched off for that character.
Player Housing, Player Ships, Mount Costomization, Farming
Focus on whatever existential threat there is at the moment isntead.