Her death is imminent. She is beyond redemption at this point. She wants nothing but power and it has been explained in the book. . 25 years and over of character development, she deserves a death touching, powerful, and climatic like Varian & Arthas' death. But who should deliver the final blow upon the Banshee Queen come 9.X?
Arthas Menethil: Sylvanas' most hated enemy. Raised her into undeath.
Reason: Poetic Justice [She dies twice by him going full circle. He made her. He unmakes her.]
Tyrande Whisperwind: People betrayed by the Horde several times. Genocide against the Night Elves. Vowed to hunt Sylvanas and will not sign the armistice until Sylvanas is deceased.
Reason: Teldrassil. Genocide. Vengeance.
Anduin Wrynn: King of Stormwind. Father unintentionally killed by Gul'dan due to Horde retreat issued by Sylvanas and her Val'kyr. Led Alliance and Horde rebels to victory in Fourth War against Sylvanas Loyalists and N'Zoth.
Reason: Fourth War. Deaths of Varian Wrynn, Calia Menethil, and Alliance w/allied Forces.
Genn Greymane: King of Gilneas. Son unintentionally killed by Sylvanas during Cataclysm. City taken by Sylvanas as well. Genocide against Worgen.
Reason: Genocide. Vengeance. Gilneas. Liam Greymane.
Baine Bloodhoof: Chieftain of the Bloodhoof Tauren. Leader on the Horde Council. Sylvanas captured Baine and issued him to death for siding with traitors. Sylvanas murdered Saurfang and Zellig.
Reason: Justice.
Thrall: Former Warchief. Put Sylvanas/Forsaken in Horde. Sylvanas killed Saurfang and attempted to kill Thrall several times.
Reason: Justice.
Nathanos Blightcaller: Love Interest of Sylvanas. Nathanos may come to his senses and become the leader the Forsaken needs.
Reason: Answering for his crimes. May kill Sylvanas out of Love.
Lich King Bolvar: Former Lich King. Killed indirectly by Sylvanas and her forces at the Wrathgate. Sylvanas broke helm of domination letting the Scourge become rampant to shatter the veil between life and the Shadowlands.
Reason: Justice. Retribution.
The rest have a lower chance of happening.