Since CoT I have believed that Silviana is Mesaana. Now that she has had a lot more screen time, I'm trying to determine if she absolutely is ruled out or not.
My reasons for this theory include:
The Mistress of Novices is a fairly powerful position, it does not have the appearance of power. Mesaana was noted in the Companion to be interested in true power, not the appearance of power.
Mesaana corrupted young people and taught them to worship the Dark One -- as MoN she would have had a perfect opportunity to influence young impressionable minds to her cause.