As the series stands, the most qualified person to take on the mantle of Darth Traya would be...the Jedi Exile.
Think about what Traya said, that "one who holds the knowledge of betrayal, who has been betrayed in heart, and will betray in turn." Think about how this applies to her:
The Exile was betrayed by Revan, believing that she was saving the Galaxy from the Mandalorian's when in fact she was being used to grow Revan's military power. The Exile was betrayed by the Jedi Order, exiled under false pretenses after willingly coming back to face their judgement. She was betrayed by Atris who wanted to use her to draw out the Sith, she was betrayed again by the Jedi Council who wanted to cut her off from the Force, and finally she was betrayed by her own mentor Kreia.
Given all that, I can easily see the Exile going dark and deciding that the galaxy can go to hell, and betraying others just like she was betrayed.