I think it rather more likely to be Rand vs. Aran'gar, with Graendal somehow escaping unharmed. Her orders from Moridin seem to line up exactly with what Jason said in his review of the book (I am repeating myself from another post, sorry about that). If Graendal can somehow trick Rand into killing Aran'gar (who, of course, isn't really a woman), then this could send Rand over the edge. Another likely scenario is Cadsuane going out in a blaze of glory - that might affect Rand that deeply, and there are no other people in his party that I can see being the one to die...it has to be a woman or he will just blow it off - but it seems more likely to be Rand actually killing a woman.

Rand killing Graendal would be nice too, but I think that Graendal will avoid a direct confrontation, because direct confrontation is not her style, and especially because she's been ordered not to harm him, but as we saw from her thoughts during the Sammael alliance, she is not averse to goading one of the other Forsaken into disobeying the Dark One's orders so that she won't have to. As we saw from Aran'gar's point of view, she very much intends to kill Rand if she ever sees him.

So, Graendal has a dual dilemma - she must carry out Moridin's orders to give Rand 'pain in his heart' or whatever it was, and she must come out on top in the 'alliance' with Aran'gar, and Graendal is smart enough to know that Aran'gar is plotting the same. The fact that Graendal's dual dilemma has the potential to be solved with one stone is not a coincidence, I don't think.