Originally Posted by
drgoll
From what I gather, the timeless isle version is if you fail to kill heroic Garrosh (basically a vision of an alternate/possible future). During the heroic Garrosh fight, all the alliance and horde leaders must be present. If you fail to kill him, he impales them all on the spikes in the harbor (as seen in the timeless isle version of the fight). This seems to be a logical step to connect the two.
I feel like this also brings the Alliance a stronger role of the story line having Stormwind be part of the fight sequence and possibly where the next Horde leader is chosen.