I'm sure if you're reading this, you've already seen quite a few of the different threads dedicated to Il'gynoths whispers. One in particular has been highly speculated.
"At the hour of her third death, she will usher in our coming."
Most often people have tried to attribute it to Sylvanas, Jaina or even Azeroth itself. Well..
That definitely makes the numbers of deaths match. And she's also one of the characters most directly involved with void and therefore the Old Gods. She also fits another of Il'gynoths quotes.
"Her heart is a crater, and we have filled it."
One of her visions as she learned to control her void powers has her one day meeting Nzoth, last of the Old Gods of Azeroth. While the visions from the void are all possible outcomes, so this is not set in stone, it's still another hint.
"She saw herself kneeling before the One Who Slumbers beneath Azeroth's waves. She saw herself killing it and taking its place, leading a throng of horrors to consume every nation."
So, maybe the big bad of Bfa won't be the Windrunner sister we suspect?