I've heard some people mention that it will be something concerning the horn of valere.

Since it's Mat, and Fain's a mystery, and Verin knows so much, it could be something Fain related.

And since she's a DF, it could be something like, "Talmanes is a DF, watch your back."

But it seems like it would be something he'd need to act upon, which she would have acted upon had she not died.

I would think it would have to fit in with her overall plan of infiltration of the DF ranks and tearing them down from the inside. From what she said to Egwene, she doesn't expect redemption, but she was doing what she could to make her DF service pay off for the good guys.