Remember this does not include:
-last minute Vader face heel turn where she takes out a Bigger Bad
-ironic punishment of being stuck as the new Jailer/other major afterlife role and being forced to fulfill a pseudo-good end
-being resurrected out of undeath magically turning her good again
All of which would be disappointing to me, but far more likely.
I still think the #1 most likely scenario is her dying in Shadowlands by our hands. They have committed for an entire expansion to make her a villain we the player hate on a personal level more than any other villain in WoW history.
That's WoW writing 101: Build up a villain that a character we hopefully sympathize with hates, then have them play some role in their ultimate defeat. (Velen + KJ, Illidan + Sargeras, Thrall + Garrosh, The Aspects + Deathwing, Tirion + Lich King, Akama + Illidan.)
This time, it's The Player + Sylvanas.