Brandon has been asked about this a lot on twitter and on tour, but cannot discuss the big mysteries from the Epilogue.
From Twitter on Jan 23, 2013:
[Asked about the Woman]
I have been advised to RAFO questions regarding most everything from there on.
This is one that I'm not answering, I'm afraid. RJ wanted some things about the ending to remain ambiguous.
Twitter transcript available on Theoryland, more detail and spoilers foor those who haven't read A Memory of Light on link.
But the most popular fan theory is that it was Nakomi, which (to my mind) has almost been confirmed by Brandon:
Who was the Aiel woman that Aviendha met on her trip to Rhuidean?
Nakomi. Also, RAFO, there's a hint in A Memory of Light.
(Later in the evening, he said that hint can be found between the
chapter "The Last Battle" and the end of the book. He also said she
came from deep in Jordan's notes, and he did not feel like he could
give more information than that. Also, she might be explained in the
encyclopedia, but no promises regarding that.)
From an interview transcript on theoryland. (Emphasis mine)
Other suggestions I've seen include:
- Tigraine / Shaiel
- The Avatar for the creator (as a version of Shadar Hairan was for the Dark One), occasionally mixed with the Nakomi idea.
I agree that the fact she isn't recognised, rules outone of the Wise Ones we've seen regularly in the series.
Unfortunately, the encyclopedia, The Wheel of Time Companion, sheds no extra light on the matter.