I don't know if this has been said, but has anyone considered Darth Bane?
"...One who holds the knowledge of betrayal, who has been betrayed in heart, and will betray in turn..."
There is a number of ways to interpret this...but here is what I think:
He was betrayed on Phaseera by his commanding officer for taking command of the Gloom Walkers...(which led him to become an apprentice of the Sith Lords)
He was betrayed in the heart by Githany..(and now that I think of it, his father)
And he went on to betray the Brotherhood of Darkness, by wiping them all out, so that the Sith could grow strong again. Most probably he will be betrayed by his apprentice, Zannah.
I don't necessarily think that this is the third Darth Traya, but I think he is a Darth Traya, seeing how his betrayals shaped the galaxy in a large way. I'm pretty sure that most true Sith could be argued as a Darth Traya, since betrayal is the essence of a Sith.