Brain Pharts 8.2.5 and Beyond.
Finish last fight in 8.3, cut scene rolls. Wrathion has found a way to clear the purge of all of the old god corruption by triggering the origination array. Unlike our encounter in Uldum, we won't avoid the full re-origination.
( screams and fade to black )
Next we wake-up back in Orgrimmar/Stormwind to find ourselves no longer level 120. Nope instead, the world has been rebuilt to a date roughly in Earth 2006. Yes, the level squish will see us back at level 60 as if we've just completed Vanilla experience.
That new level 60 content won't be added to Vanilla. It will be the next expansion where we don't repeat our mistakes in Burning Crusade.