Hmm... Another Darth Traya. I personally think that the next one will likely be Revan, or the Exile, or maybe the new PC (if there is one). There is a small possibility that it is one of the party-members (like Bastila) or the remaining Jedi but by how Kriea always treated them... I doubt it. Such roles are generally saved for the main characters anyway.
About there always being a Traya: metaphor for the betrayer of every era, of every story. Anakin/Vader, Ulic, Jacen/Caedus. The list goes on.