Imagine that the timeline crashes next patch. There's a cinematic displaying things going from slowly settling to peace, to the Alliance and Horde shattered and events turning out totally differently. Sylvanas is laying siege to Stormwind, Jaina has wiped Orgrimmar off the map, Thunder Bluff is a smoking husk still getting bombarded by the gnomes and dwarves from the air. Teldrassil burns under the weight of a blood elf attack. Half the Eastern Kingdoms has been reduced to a blighted wasteland.
Gameplay picks up with us in our respective capitals (the Ruins of Orgrimmar and Besieged Stormwind, temporarily-phased versions of the cities proper), where we're contacted by Timewalkers with an urgent message. We travel to the Caverns of Time and go through a series of solo secenarios, group scenarios, and an event a la the Battle for Light's Hope detailing us doing everything we can to settle the wars down, figure out what the hell happened, and find the points of divergence--all of which turn out to be key moments in history that have been wildly altered by agents of the last, unnamed Old God, the same one fated to drive Nozdormu mad and bring about the birth of the Infinite Dragonflight. With the last of her power, Soridormi sacrifices herself to open a gateway into the time stream, wherein we use techniques devised by the Timewalkers on the Timeless Isle to travel to those periods in history, which comprise the expansion's 'zones,' as well as a massive continent on Draenor itself which contains the endgame factions and raid content. Uninstanced Azeroth, the one we play in now, can be explained as the Bronze Dragonflight sending us to an identical timeline which is only different in that our character in specific was never born, meaning someone else took up the banner in our place (Varian/Vol'jin?).