The Caverns of Time are a pretty neat place. With the recent announcement of Old Stratholme, we'll get to go back and preserve the timeline through four different events: Medivh's opening of the Dark Portal, Thrall's escape from Durnholde Keep, Arthas's purging of the city of Stratholme, and Azeroth's victory at the Battle for Mount Hyjal. The mysterious Infinite Dragonflight is trying to disrupt the timelines for unknown reasons, and we have to keep things the way they were.
But why? Yes, any change in the past will have a ripple effect on the future, bla bla bla, but there are people who would definitely prefer that events went down differently in three of the four CoT instances. Blizzard doesn't show us what happens when we wipe, fail or simply choose not to preserve things in CoT ... so we've had to fill in the blanks ourselves.
The Battle for Mount Hyjal: Okay, no one really wants to see history changed for this one. Heck, it even looks like the Infinite Dragonflight may not want to change it, since we see none of their members in the battle. If history was changed ... actually, I'm not sure what the Burning Legion really would do if they won the war. Destroy all the mortal races in their quest to bring chaos to the universe? Enslave all the playable characters and force them to become an invading force for another world? Man, that would be awesome. For the Legion. Not so much for Azeroth. The Burning Legion winning at Hyjal is like a big Game Over sign for the Warcraft universe.