I kinda wanted to start a discussion about what the legion's plan has been so far. If we start the new story arch back to MoP when Wrathion predicted the return of the legion. And consider what Turalion and Alleria have to say when we meet them again.
So far we know that for some reason, an infinite dragon to be, took garrosh to AU Draenor, with the intention to create the Iron Horde. However, it's also very possible that he took Garrosh to AU Draenor with the sole intention of getting Guldan back.
Does this mean that the legion has some serious influence over the infinite dragonflight? It is possible to give legion explanations to every Caverns of time zone.
So, in short. Legion gets bronze dragon to open a portal, so they can get Guldan to our Universe, because I imagine our Azeroth is special in some way.
That being said, I imagine that the news we will be getting is that the legion is infinite. Which begs the question, why do they care so much about our Azeroth? I imagine it has something to do with the old gods, but who knows.