This comment intrigued me. The last time the Legion attacked Azeroth, they made a lot of lasting changes to Kalimdor and the Eastern Kingdoms. Part of Ashenvale became Felwood. Lordaeron was taken over and eventually fell into the hands of the Forsaken, who were part of the Legion during most of WarCraft III. That invasion was huge... and if this next one is BIGGER, then maybe... just maybe...
... we will see another Cataclysm-style revamp of the two core continents. Maybe not every zone, but that would make sense. They also said that longtime players will be "shocked." That would certainly shock us. It would also fit a pattern: Azeroth and Kalimdor were modified after two expansions. Legion will be two expansions after the last modification. A lot of zones from Cataclysm could certainly use another update.
This is purely conjecture on my part.