Looking over a lot of threads, there seems to be an assumption that we're redoing Mists of Pandaria, with Sylvanas as Garrosh, and Saurfang as Vol'jin. And then Sylvanas becomes a raid boss and is deposed by the Alliance/Saurfang at the end of the expansion.
What if this doesn't happen? Sylvanas remains Warchief, with the rest of the Horde rallying behind her, maybe reluctantly in some cases. Saurfang remains outcast, or becomes reconciled to the Horde. The Alliance is unable to secure a full/major victory against the Horde this expansion.
There's an argument that the big mistake Garrosh made was making enemies of the non-orcish Horde. So far, Sylvanas is not doing that, but is working with the entire Horde.
I think Blizzard might take this path, as it would make the faction war more or less permanent for future expansions. It also avoids killing off a fan favorite character, and avoids another round of Name the Warchief. It would also avoid completely rehashing MoP.