"Killing major characters is inevitable when progressing the story and never easy. Not everyone has a heroic death or one that comes unexpectedly."
Quote from the Warcraft Story Crew at E3.
To me it seems like more confirmation that she won't make it through the expansion.
Sidenote, from the comic with the the windrunner sisters, the void keeps trying to get Alleria to kill Sylvanas, I think that ties back to the Illgynoth quote.
"At the hour of her third death, she will usher in our coming"
So well probably see Sylvanas' death as a precursor of sorts to next expansion, another question to be explored, why does her death usher in the old gods? Or do they just know that after Sylvanas dies that they are very close to being released?