Right, this is just speculation, but I've been adding up a lot of recent reveals blizzard has shown, and also taken a theme from one of the previous novels they they explored, and thought maybe I figured out what it will.
The timeless isle has to it many visions that shows us things that seem to question outcomes and doesn't make a whole lot of sense, but maybe infact, it does.
Vision of stormwind destoryed and all faction leaders dead
Newest vision Anduin seeing an orc in a cage in temple of the white tiger, said to be garrosh from data files.
A traitor bronze dragon.
I think what we might be seeing are visions of alternate realities, or maybe one alternate reality, which is basicly a timeline that exists because of a certain event not happening at a period of time, and so creating another timeline from it.
given how the theme of the timeless isle, and the involvement of the bronze dragonflight are involved, we might be seeing what the next expansion theme is, an alternate reality of azeroth, where all the leaders are dead, and maybe in that one garrosh somehow survives.
Now before you say 'alternate timeline stories don't work in wow!', yes, they do and have already been used as example. In Thrall: Twilight of the aspects, Thrall is attacked by an alternate reality version of Blackmoore, sending Thrall into a timeway that brings him to a timeline where he had died as a baby, and the result lead to blackmoore becoming a powerful figure, killing doomhammer, and eventually being given time hopping powers by Murazond, in order to try and kill Thrall. He follows Thrall to our timeline, where Thrall kills him in dragonblight.
So, knowing that blizzard are prepared to do alternate reality timelines, and that the bronze dragonflight having a traitor in it, and the visions of time we've seen, to me, it suggests we're be exploring an alternate timeline in the next expansion.
This is interesting, since one of the comic book series I liked was Exiles, in which an alternate reality version of sabertooth (who as a good guy) blink, and several others from the age of apocalypse universe travel though different timeways to fix problems created in those alternate realities. Given blizzards track record of being very comic bookish in there design, this wouldn't surprise me as a theme.