First we have to make 1 thing clear.
Who ever "she" is, her death need to have an effect that ushers in the coming of the old gods/void lords.
Thats why Sylvanas and Jaina are already out for me.
I first thought this quote fits really good to Sylvanas. But she died already 3 times and even if we say, the Banshee Queen died only 2 times, it still doesnt make sense, why exactly Sylvanas her dead would usher in the coming of the old gods/void lords.
If she dies she is just "dead" and her soul in that hell she once saw. Nothing more and nothing less.
I have 3 candidates that makes the most sense for me.
Candidate number 1: Alleria (most likely)
Alleria Windrunner: Indeed. Sometimes I feel that I have already died twice. Once when we were presumed lost on Draenor, and again when gripped by the shadow of the fallen naaru. || also we have a vision where Alleria sees herself how she takes Nzoths place and kills her own Son
salty dude:"This is a red herring, mark my words. Alleria won't be the one who ushers in their coming, because it would be way too obvious and boring."
I disagree. Blizzard showed more then once that the most obvious way is blizzards way.
Candidate number 2: Azeroth (not really sure if blizzard would kill Azeroth)
Candidate number 3: Azshara (most unlikely, since her death wouldnt have much of an effect except the release of Nzoth?)
Im sure that who ever it is, it will be at the latest of BfA (8.3) be known.