On a more serious note, if I had to make a real prediction, then I’d guess that Sylvanas isn’t going to be killed by one vengeful enemy; she’s going to fall to the power of friendship.
Because she’s a sociopath and can’t relate to the non-narcissist living, Sylvanas consistently underestimates the extent to which people will stay loyal to and fight for unselfish ideals–for loved ones, for the Horde, for the Alliance, etc. So she’ll develop some super-villain scheme that relies on her enemies acting on hatred–like pitting Tyrande and Genn against Anduin or something, or using her loyalists to sew distrust between the Horde and the Forsaken.
And her plan just…won’t work. They might get their panties in a knot, but ultimately the heroes of Azeroth will learn to forgive and forget, because Sylvanas doesn’t understand them 20% so we’ll as she thinks she does. Her enemies will collect the elements of harmony and prepare to blast her into oblivion with a rainbow ion cannon.
She won’t even see it coming because she’ll have assumed that manipulating the “tin soldiers” was the easy part of her plan. Nathanos will probably see it coming and try desperately to warn her, but by then she’ll be so mad with ultimate power that she’ll just see this as disloyalty and kill him, sealing her unicorn fate.
IOW, she’ll just die at the end of the war campaign in the BfA 2.0 expansion.