Personally, I have always thought of Sylvanas as that strong-willed, individualist character. She stubbornly defended Silvermoon, she broke free from the Lich King's control because of how strong her spirit was, and then had the gall to openly disobey both Garrosh and Vol'jin. The implication that Sylvanas is now merely a minion to some death god feels out of character for her.
I only hope that she will eventually turn on that new master of hers as well. Ideally, she should fail and die (karma, bitch), but if she became some sort of death goddess for us to fight, it'd be pretty cool as well.