First of all, I don't believe they're gonna kill off Sylvanas, sadly.
Second of all, I don't think Tyrande should be the one to kill her off, yes her tree got burnt down but does that automatically mean she get to land the killing blow? Nah.
Personally I want it either to be Zovaal, or the spirits of the past, e.g. Saurfang, Arthas, Varian, Random Nelf spirit e.t.c. all ganging up on her.